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Abstract.-The photochromic compounds N-p-phenylazophenyl-N-phenylcarbamylcholine chloride and p-phenylazophenyltrimethylammonium chloride inhibit the carbamylcholine-produced depolarization of the excitable membrane of the monocellular electroplax preparation of Electrophorus. The trans isomer of each predominates in the light of a photoflood (420 mMA) lamp; they are stronger inhibitors than the cis isomers, which predominate under ultraviolet (320 mp) irradiation. The potential difference across the excitable membrane may be photoregulated by exposing an electroplax in the presence of a solution of carbamylcholine and either of the two compounds to light of appropriate wavelengths, since light shifts the cis-trans equilibrium. The system may be considered as a model illustrating how one may link a cis-trans isomerization, the first step in the initiation of a visual impulse, with substantial changes (20-30 mv) in the potential difference across an excitable membrane.The electrical currents which propagate nerve impulses are carried by ion movements resulting from changes in the ionic permeabilities of excitable membranes. It has been proposed that such permeability changes are effected by a series of reactions in which acetylcholine released by stimulation acts as a trigger. According to this hypothesis, combination of acetylcholine with the acetylcholine receptor leads to excitation, perhaps through regulation of the ionic permeabilities of the excitable membrane by calcium ions (known to be associated with excitability) released by a conformational change of the receptor. Rapid hydrolysis of acetylcholine by acetylcholinesterase would allow the return of the receptor to its resting state and the reestablishment of the barrier to ion movements.'1 2 Initiation and propagation of nerve impulses can result from the response of sensory receptors to specific stimuli, such as light, sound, and touch. A great deal is known about the physiology of the receptor cells, and information is being accumulated concerning the elementary reactions occurring in the reception of stimuli. Vision, for example, is based on the cis-trans isomerization of retinal.3 Cis-retinal reacts in the dark with the protein opsin to form rhodopsin. Lightinduced isomerization of the retinal to the all-trans conformation leads to nerve excitation. Absorption of only a few quanta of light is sufficient to produce a measurable response in the retina.4 However, the mechanism by which isomerization leads to excitation remains open to speculation. A model system is presented here in which the potential difference across the excitable membrane of the 1230

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