Neurology, Queen Square, LondonBiochemical data suggest that L-glutamate (L-glu) and L-aspartate (L-asp) could be the neurotransmitters of the parallel and the climbing fibres respectively in the mammalian cerebellum. We have now used in vitro sagittal slices of adult rat cerebellum to study the responses of Purkinje cells (PCs) to ionophoretic applications of L-glu and L-asp in the molecular layer.Extra-and intra-cellular recordings were made from eighty PCs and L-glu and L-asp were released into their dendritic fields by short current pulses (5 to 100 msec) through a separate multibarelled electrode filled with 0 5 M salt solutions of the drugs.In the nineteen cells recorded from intracellularly, transient depolarizations up to 17 mV in amplitude and with depolarizing slopes up to 0 4 mV/msec were evoked by 2 to 10 nC L-glu or L-asp applied in the molecular layer. Maximum sensitivities were 8 mV/nC for L-glu and 6-4 mV/nC for L-asp. These depolarizations were accompanied by no change in cell input resistance, whereas steady applications of high doses of L-asp or L-glu decreased the membrane resistance.In the extracellularly recorded cells, the lowest threshold for L-glu or L-asp mediated excitations of PCs was as low as 25 pC, with a latency for the response as short as 7 msec. The mapping of these excitatory responses confirmed that the sensitivity of the dendrites for L-glu was greater than that of the soma (Chujo, Yamada & Yamamoto, 1975), and we showed in addition that the same was true for L-asp. No differential sensitivity of PC dendrites for L-asp and L-glu was detected in 66 0 of the tested cells; in the remainder, the sensitivity for L-asp declined markedly in the upper third of the molecular layer, whereas it remained high for L-glu.Finally, the agonist quisqualic acid was a much more potent excitant of PCs than L-glu or L-asp when applied in their dendritic fields, whereas the other major agonist N-methyl DL-aspartate (Sigma) only produced a very weak excitatory effect under the same conditions. The excitatory effects of L-asp, L-glu and quisqualic acid on PCs were antagonized by L-glutamic acid diethyl ester more consistently than by D-a amino adipate, suggesting, together with the previous observations, that L-asp and L-glu act on PCs via quisqualic acid receptors rather than via N-methyl D-aspartate receptors.