We stimulated the aortic chemoreceptors in dogs that were anesthetized with chloralose and artificially ventilated by perfusing the isolated aortic arch with venous blood. Inotropic responses were determined by measuring the maximum rate of change of left ventricular pressure (dP/dt max) with aortic pressure and heart rate held constant. Stimulation of the aortic chemoreceptors resulted in an average increase in heart rate of 14 ± 2.0 beats/min (mean ± SE) from 166 ± 7.7 beats/min and an increase in dP/dt max of 501 ± 85 mm Hg/sec from 3508 ± 154 mm Hg/sec. These changes were statistically significant (P< 0.001). The afferent pathway of the reflex was shown to be in the vagus nerves and the efferent pathway in the cardiac sympathetic nerves. In some of the dogs, the carotid chemoreceptors were also stimulated. This resulted in decreases in heart rate and dP/dt max of 48 ± 24 beats/min and 795 ± 142 mm Hg/sec. Thus we have shown that stimulation of aortic chemoreceptors evokes chronotropic and inotropic responses opposite to those evoked from stimulation of carotid chemoreceptors. Circ Res 46: 77-83, 1980