Iontophoretic techniques were used to examine the effect of dopamine on glutamate-induced excitation and gamma-aminobutyric acid (GABA)-induced inhibition of single striatal neurons in rat brain. When dopamine was applied at concentrations that produced little or no inhibition of spontaneous firing rate, both glutamate-induced excitation and GABA-induced inhibition were enhanced. In contrast, when dopamine was applied at doses that significantly decreased spontaneous firing, glutamate-induced excitation was greatly reduced, though GABA-mediated inhibition remained enhanced. Thus, dopamine acts to modulate the efficacy of other neurotransmitters impinging on striatal neurons, but has a qualitatively different effect on the excitatory activity of striatal cells depending on its concentration.