1. Dogs were anaesthetized with chloralose and artificially ventilated. The regions of both carotid bifurcations were vascularly isolated and perfused at constant pressure with arterial blood or with venous or hypoxic blood. 2. Inotropic responses were assessed by measuring the maximum rate of change of left ventricular pressure (dP/dtmax) with aortic pressure and heart rate held constant. 3. Stimulation of the chemoreceptors with venous blood from the inferior vena cava resulted in a decrease in dP/dtmax from 510 +/- 27 kPa.s-1 to 418 +/- 21 kPa.s-1 (mean +/- S.E.): a change of 17.9 +/- 1.0% 4. In experiments in which the oxygen tension of the blood perfusing the carotid chemoreceptors was decreased in steps between 10 and 3 kPa, by use of an oxygenator, graded responses of dP/dtmax were obtained at each step. 5. The inotropic responses to chemoreceptor stimulation were abolished by raising carotid pressure. 6. The inotropic responses were abolished either by crushing both carotid bodies or both ansae subclaviae, indicating that the reflex originates from the carotid bodies and that the efferent pathway is in the cardiac sympathetic nerves.