Arterial blood pressure and heart rate were recorded from male Wistar rats anaesthetized with urethane. Intravenous injection of capsaicin, 1 microgram, produced a reproducible triphasic effect on blood pressure, comprising an initial fall in blood pressure and heart rate, followed by a transient and then a sustained pressor response. The depressor response and bradycardia were abolished by vagal section. The transient pressor response was altered in shape by hexamethonium. Slow intravenous infusion of capsaicin, 50 micrograms over 12 min, produced only a sustained pressor response accompanied by tachycardia, which was resistant to hexamethonium but abolished by morphine and pithing. Responses to both 1 microgram injection and 50 micrograms infusion of capsaicin were unaffected by the SP antagonist, spantide, but were abolished by capsaicin pretreatment of the rats. Capsaicin induces complex effects on the cardiovascular system, the nature of which varies with the dose and speed of administration.