1 The injection of dopamine (100 to 500 microgram) into the cerebral ventricles (i.c.v.) of 10 unanaesthetized dogs produced a dose-dependent increase in arterial blood pressure and heart rate. The dogs licked, swallowed, sometimes vomited and became sedated. 2 Autonomic ganglion blockade with hexamethonium (10 mg/kg i.v.) abolished cardiovascular responses to i.c.v. dopamine, indicating that dopamine was exerting its effect within the central nervous system. 3 The dopamine receptor antagonists, haloperidol (500 microgram), chlorpromazine (200 micrograms) and ergometrine (500 micrograms), each given i.c.v., sugsequently abolished the cardiovascular responses to dopamine. 4 Pretreatment with either the beta-adrenoceptor antagonist, propranolol (600 microgram) or the alpha-adrenoceptor antagonist, phentolamine (1 mg) given i.c.v. had no significant effect on the response to dopamine. 5 It is suggested that dopamine injected into the cerebral ventricles of the unanesthetized dog causes hypertension and tachycardia by activating central dopamine receptors.