The excitatory response to motor cortex stimulation of 201 striatal neurones was recorded electrophysiologically to test the effects of melatonin (aMT) and/or D1 and D2 antagonists. Iontophoresis of aMT attenuated the excitatory response in 68.5% of neurones, with a latency of 2-4 min and enhanced the excitatory response in 11.9% of the neurones; 19.6% showed no change in response. Iontophoresis of sulpiride (D2 antagonist) produced an immediate increase in the excitatory response in 62.8% of neurones, an attenuation in 2.3% and no change in the response of 34.9%. The ejection of sulpiride counteracted the aMT-dependent inhibition of the excitatory response of striatal neurones. SCH-23390 (D1 antagonist) iontophoresis had no significant effect. The results show that the same striatal units may be driven by aMT and D2 receptors. However, the significant difference in the latency of the responses suggests that the effects of these two substances are mediated by different receptor/intracellular messengers.