The electrophysiological effects produced by different concentrations of kainic acid (KA) were studied by utilizing intracellular recordings from neostriatal slices. In most of the recorded cells (81%), concentrations of KA ranging between 10 and 300 nM produced reversible and dose-dependent membrane depolarizations. Higher concentrations of this agonist caused larger depolarizations and changes of the membrane properties of the recorded neurons not reversible during the time of recording. In a smaller percentage (19%) of the recorded cells, 10-100 nM KA did not produce significant membrane depolarizations; in these neurons, the depolarizations produced by higher concentrations of KA were small and reversible. The 2 populations of neurons showed similar electrophysiological properties and did not reveal differential sensitivity to quisqualic acid (QUIS; 10-30 microM) or to NMDA (10-30 microM). Tetrodotoxin (TTX; 1 microM) did not reduce the depolarizations produced by KA and by NMDA. Low-calcium, cobalt-containing solutions abolished the effects produced by NMDA, but not the KA-induced depolarizations. Kynurenic acid (500 microM) significantly antagonized the depolarizations produced by KA and reduced the changes of the membrane properties caused by high doses of this agonist. In several neurons, KA induced bicuculline-sensitive synaptic depolarizing potentials. Our findings suggest the presence of 2 subpopulations of neostriatal neurons showing differential postsynaptic sensitivity to KA. The differential sensitivity of neostriatal neurons to KA might influence the responses of these cells to glutamatergic cortical inputs and the degenerative changes observed in neostriatal neurons in some pathological conditions.