Electrical stimulation of a discrete region of the rabbit brainstem, in the lateral hypothalamic area near the mammillothalamic tract, caused an increase in arterial blood pressure accompanied by bradycardia in anaesthetized animals. The cardiac output, measured by a thermodilution technique, was found to fall when the stimulus strength was great enough to evoke strong bradycardia. Threshold current for this effect varied in different animals in the range: 50-350 microA (1 msec pulse duration and 60 pulses/sec repetition rate). Bilateral vagotomy eliminated almost all the negative chronotropic effects of hypothalamic stimulation, but reversed only about one-third of the fall in cardiac output during strong stimulation. It is suggested that negative chronotropic, and possibly inotropic, vagal effects on the heart play a relatively minor role in reducing cardiac output. The major factor in causing output to fall may be left ventricular overload during the widespread peripheral vasoconstriction that accompanies stimulation of this region of the rabbit hypothalamus.