The effect of microelectrophoresis of glutamate on spontaneous activity of sensomotor cortical neurons located 80-250 μ from the point of application of glutamate was studied in cats anesthetized with pentobarbital. If glutamate was applied at distances of under 100 μ from the neurons the predominant response was one of excitation, evidently due to the direct action of the excitatory mediator. With more distant application inhibition of spontaneous activity predominated: at distances from 100 to 200 μ it was observed in 57%, and between 200 and 250 μ, in 70% of cases. Application of picrotoxin close to the neuron weakened inhibition induced by microelectrophoresis of glutamate through a neighboring microelectrode.