Twenty chronically prepared, unanesthetized rabbits received both high-frequency (200 pulse/sec), short pulse-train (1.0 sec) and relatively low-frequency (25 pulse-sec), long pulse-train (10 sec) electrical stimulation of the hypothalamus. High-frequency, short pulse-train stimulation elicited a pressor response and bradycardia at all 27 electrode sites. In contrast, three other cardiovascular response patterns were obtained following low-frequency, long pulse-train stimulation. These latter patterns reflected a medial-lateral organization of autonomic function within the hypothalamus. Whereas all 15 lateral hypothalamic placements yielded depressor responses, 7 of 12 medial hypothalamic placements yielded pressor responses and tachycardia. Cardiovascular changes following administration of selective autonomic blocking agents (e.g., phentolamine, propranolol, methylatropine) suggest that high-frequency, short pulse-train stimulation elicited a pressor response followed by a reflexive bradycardia essentially mediated by an increase in vagal restraint. In contrast, the heart rate changes observed to low-frequency, long pulse-train stimulation all appear to have been importantly influenced by changes at the heart in beta-adrenergic activity.