GABA, applied by iontophoresis to striatal neurons of the rat in an in vivo preparation, depolarized the membrane potential and decreased the input resistance in a dose-dependent manner. The null potential of the GABA depolarization was about -50 mV. In addition, a fading of the GABA-induced response was observed for prolonged and relatively high amino acid application. We conclude that GABA has a depolarizing effect on striatal cells. This is in line with recent in vitro works describing a depolarizing effect of GABA in the rat neostriatum.