The effect of ketanserin on sympathetic efferent nerve activity which was recorded from the postganglionic renal nerve, inferior cardiac nerve and preganglionic adrenal nerve was studied in buffer nerve-intact rats. Intravenous administration of ketanserin (0.05-5.0 mg/kg) produced bradycardia and a reduction of mean arterial pressure dose-dependently. Renal nerve activity and inferior cardiac nerve activity also were reduced by ketanserin. After i.v. administration, baroreceptor afferent nerve activity did not increase, but actually decrease. Moreover, i.v. and i.c.v. administration of ketanserin caused a decrease in preganglionic adrenal nerve activity accompanied with hypotension and bradycardia. Ketanserin (0.5 mg/kg i.v.) and clonidine (3 micrograms/kg i.v.) produced equihypotension and bradycardia. On the other hand, the ketanserin-induced reduction of renal nerve activity was milder than that induced by clonidine. The present findings confirmed that ketanserin has a central sympathoinhibitory action. This central action of ketanserin may play an important role in ketanserin-induced bradycardia and may be partially responsible for ketanserin-induced hypotension.