(,r = 75-200 ms). No filtering was discerned in the transfer of dim-flash responses from rods to bipolars. On average, horizontal cells were five times as sensitive (mV/ Rh*) and hyperpolarizing bipolar cells 10-7 times as sensitive as their paired rods.3. For brighter flashes, up to 1600 Rh*, the rising and return phases of bipolar responses appeared to be simple scaled versions of the rod responses. The scaling factor was equal to the ratio of flash sensitivities for dim flashes. Rod responses greater than about 2 mV produced a saturation of the bipolar cell response.4. The return phases of the horizontal cell responses were kinetically similar, scaled versions of the rod responses for rod potentials less than about 5 mV. However, the rising phases lagged significantly behind those of the rod. The effective time constant of the lag increased proportionally with flash intensity. For the brighter flashes, the horizontal cell response peaked as much as a second after the rod response.5. The linear scaling, minimal temporal filtering and saturation of the bipolar cell responses were satisfactorily reproduced by a model of synaptic transfer that assumed that the rate of transmitter release followed the rod voltage exponentially and that the postsynaptic conductance followed Michaelis-Menten saturation (Falk & Fatt, 1972).6. The progressively longer lag in the horizontal cell responses to brighter flashes was satisfactorily simulated by a kinetically limited Falk and Fatt model which postulated that the effective electrical time constant of the horizontal cell membrane strongly depended on synaptic or voltage-modulated conductances. 7. Satisfactory model simulations of all postsynaptic responses required that an e-fold change in the release rate of transmitter from the rod be obtained with a 2 mV change in the rod potential.

SUMMARY1. Intracellular responses were recorded from on-centre and off-centre ganglion cells in isolated eyecups of the mudpuppy, Necturus maculosus.2. Current-voltage relations were measured in darkness, during illumination of the receptive field centre, and after chemically mediated synaptic inputs were blocked by 4 mM-cobalt chloride.3. In on-centre cells the membrane potential in darkness was -56 + 6 mV (mean+ S.D.). Addition of Co2+ resulted in an average depolarization of 10 mV and an average decrease in conductance of 2-1 nS. These results suggest that in darkness on-centre cells are tonically inhibited by synaptic input which increases conductance and has a reversal potential more negative than the dark membrane potential.In off-centre cells the membrane potential in darkness was -46 + 5 mV. Addition of Co2+ caused an average hyperpolarization of 6 mV and an average decrease in conductance of 1-5 nS. These results suggest that in darkness off-centre cells receive a tonic excitatory input which increases conductance and has a reversal potential more positive than the dark membrane potential.4. In on-centre cells light causes a sustained depolarization. This response involves an increase in a tonic excitatory input which increases conductance and has a reversal potential more positive than the dark membrane potential.5. In off-centre cells, light causes a sustained hyperpolarization. This response involves an increase in a sustained inhibitory input which increases conductance and has a reversal potential more negative than the dark membrane potential.6. The depolarizing off-response of off-centre cells is associated with an increase in an excitatory input which increases conductance and has a reversal potential more positive than the dark membrane potential. This response may be due to a temporary increase in the excitatory input which is tonically active in darkness or may reflect an additional excitatory input.7. It is suggested that in both on-and off-centre ganglion cells the balance of sustained excitatory and inhibitory synaptic inputs determines the resting potential * Present address:

SUMMARY1. Transient and sustained inhibitory synaptic inputs to on-centre, off-centre, and on-offganglion cells in the mudpuppy retina were studied using intracellular recording in the superfused eye-cup preparation.2. When chemical transmission was blocked with 4 mM-Co2+, application of either glycine or y-aminobutyric acid (GABA) caused a hyperpolarization and conductance increase in all ganglion cells. For both amino acids, the responses were dose dependent in the range 0-05-10 mm, with a half-maximal response at about 0-7 mM. Glycine and GABA sensitivities were very similar in all three types of ganglion cells.3. The response to applied glycine was selectively antagonized by 10-5 M-strychnine and the response to applied GABA was selectively antagonized by 10-5 M-picrotoxin.4. In all ganglion cells, 10-5 M-strychnine eliminated the transient inhibitory events which occur at the onset and termination of a light stimulus. The block of transient inhibition was associated with a relative depolarization of membrane potential and decrease in conductance at these times. Strychnine had no effect on membrane potential or conductance in darkness or during sustained inhibitory responses to light.5. Picrotoxin (10-5 M) did not block transient inhibitory events in any ganglion cells, but did affect other components of their responses.6. The results suggest that in all three classes of ganglion cells transient inhibition, but not sustained inhibition, may be mediated by glycine or a closely related substance.

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SUMMARY1. Synaptic inputs to on-off ganglion cells in mudpuppy retina were studied by measuring current-voltage relations in darkness, during different phases of the response to light, and in the presence of 4 mMCo2+.2. The addition of Co2+ to the bathing medium usually caused a hyperpolarization of the membrane potential in darkness and an increase in input resistance, indicating that on-off ganglion cells receive tonic excitatory synaptic input in darkness. Other results suggest that an additional synaptic input, with a reversal potential near the dark potential, may also be active in darkness.3. At the onset of a light stimulus in the receptive field centre all on-off ganglion cells responded with transient excitatory and inhibitory synaptic events, both of which were due to increases in conductance. Similar transient excitatory and inhibitory events occurred at the termination of the light stimulus.4. In about one-halfofthe on-off ganglion cells studied the synaptic activity during steady illumination was the same as in darkness. In the remaining cells steady illumination caused an increase in sustained inhibition.