Injection of a small dose of yohimbine, an alpha 2-adrenoceptor blocking agent (0.1 mg X kg-1 i.v.) increased systolic blood pressure, heart rate and cardiac performance in anaesthetized dogs. A larger dose of this drug (1 mg X kg-1 i.v.) influenced the cardiovascular system in an opposite way. The tachycardiac effect of yohimbine was also observed when the drug was administered either into the vertebral artery or the cisterna magna of chloralosed dogs. The increase in heart rate was found to be due to an increase in sympathetic tone and to a decrease in vagal tone. Some data reported here suggest that yohimbine could impair central cardiovascular regulation by acting on the baroreceptor reflex pathway. Indeed, intracisternal administration of yohimbine at small doses (1) reduced the tachycardiac and pressor responses to carotid occlusion, (2) reduced the bradycardia produced by intravenous noradrenaline. It is also suggested from the results of this investigation that yohimbine decreases the vagal part of the bradycardia resulting from the stimulation of the aortic baroreceptors but not those resulting from the stimulation of carotid sinus baroreceptors. Intracisternal administration of yohimbine also produced a significant inhibition of the Bezold-Jarish reflex induced by intravenous veratridine.