Ritanserin (RIT), widely-used as a selective 5-HT2A/2C receptor antagonist, has been reported to produce significant therapeutic effects on the negative symptoms of schizophrenia and to improve extrapyramidal side effects induced by neuroleptics. Because midbrain dopamine (DA) systems are believed to be the major site of action for many antipsychotic drugs, the effect of RIT on substantia nigra DA neurons was examined in chloral hydrate-anesthetized rats using single unit recording techniques. Systemic injection of RIT (0.1-6.4 mg/kg, i.v.) had no consistent effect on basal firing rate but significantly reversed the inhibition induced by both direct and indirect DA agonists. However, our data suggest that this effect of RIT is largely mediated by a mechanism independent of 5-HT. Thus the 5-HT2A/2C agonist 1(2,5 dimethyoxy-4-iodophenyl)-2-aminopropane showed no effect on either basal firing rate or the inhibition induced by the direct DA agonist quinpirole. Neither the selective 5-HT2A antagonist MDL 100907 nor depletion of endogenous 5-HT using p-chlorophenylalanine mimicked the effect of RIT (i.e., attenuated quinpirole-induced inhibition). Furthermore, the effect of RIT persisted in animals pretreated with p-chlorophenylalanine. Because RIT is known to bind D2-like receptors and because the inhibition of DA neurons induced by low doses of a direct DA agonist is believed to be mediated by DA autoreceptors, these results suggest that RIT may act on DA autoreceptors directly as a DA antagonist. Since similar doses of RIT were reported to have no significant effect on postsynaptic D2 receptors in the striatum, it is possible that RIT at the doses used may selectively block DA autoreceptors.