Single neuronal activity was recorded extracellularly in the substantia nigra pars compacta (SNC) in rats anesthetized with chloral hydrate. Haloperidol in a dose of 10 micrograms/kg had no significant effects on the SNC neurons. R(-)apomorphine (cumulative i.v. dose of 40 micrograms/kg, given as: 5, 5, 10 and 20 micrograms/kg) inhibited the firing rate of dopaminergic neurons, in a dose-dependent manner. Haloperidol (cumulative i.v. dose of 10 micrograms/kg, as: 2.5, 2.5 and 5 micrograms/kg) reversed the effect of apomorphine. Complete reversal of the firing rate to haloperidol was observed with a dose of 10 micrograms/kg. In rats pretreated with 10 micrograms/kg of haloperidol, there was a dramatic shift to the right of the apomorphine dose-response curve (cumulative i.v. dose of 800 micrograms/kg, as: 50, 50, 100, 200 and 400 micrograms/kg), and this inhibition was reversed by haloperidol (cumulative i.v. dose of 400 micrograms/kg, as: 50, 50, 100 and 200 micrograms/kg). Apomorphine in doses of 40 micrograms/kg and 800 micrograms/kg elicited yawning behavior and stereotypy, respectively. Apomorphine in a dose of 800 micrograms/kg elicited stereotypy in rats treated 3 min before with 10 micrograms/kg of haloperidol. Therefore, electrophysiological determinations of events in the SNC dopaminergic neurons are given support by the behavior observed in these rats.