Experiments were done to characterize the vascular responses to stimulation of aortic and carotid chemoreceptors and to identify the efferent components of the sympathetic system which are activated in different vascular beds. The chemoreceptors were stimulated with nicotine and cyanide in anesthetized and artificially ventilated dogs. The gracilis muscle and hindpaw were isolated and perfused with blood at constant flow. Changes in perfusion pressure reflected changes in total vascular resistance, and changes in small vein pressure reflected changes in venous resistance. The results indicate that stimulation of carotid and aortic chemoreceptors activates selectively efferent adrenergic constrictor fibers supplying prevenous resistance vessels in the gracilis muscle and venous resistance vessels in the paw. In contrast, there was a dilatation of prevenous resistance vessels in the paw caused by activation of efferent sympathetic dilator fibers and not by withdrawal of sympathetic constrictor tone. The dilatation was not mediated through the release of acetylcholine, histamine, or bradykinin nor through beta receptors. Bilateral denervation of the carotid sinus and body and bilateral vagotomy abolished the reflex responses caused by injections of the chemicals. These responses were not the result of activation of baroreceptors since they were not reproduced during electrical stimulation of the carotid sinus nerve.

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