In a previous study it was found that hypercapnia increased venous return (Q) and mean systemic pressure (MSP) in dogs on a cardiac bypass. This response, if present in a diving mammal, would indicate some basic similarities in the mammalian peripheral circulation. Three seals (25 kg) were initially tranquilized with Ketalar (7mg/kg), then anesthetized with Pentobarbital sodium (40mg/kg) prior to thoracotomy and right heart bypass. Right atrial pressure was kept constant at zero by adjusting the height of a Starling resistor. Ventilation with 10% CO2 elevated P(a)CO2 from 30-75 mmHg, and increased Q 17%, MSP and P(a) (art.press.) both by 7%. Ventilation with 8% O2 in the inspired air did not change Q, but increased MSP and P(a) by 10%. Left atrial pressure remained constant throughout. After complete peripheral chemodenervation (carotid and aortic bodies), CO2 inhalation increased Q 30%, while 8% O2 decreased Q by 9% in one animal. Administration of dibenzyline (1mg/kg) to the normal animal blocked the typical CO2 response, indicating that it is mediated by catecholamines. The importance of this response in a diving mammal is uncertain since cardiac factors and other specialized circulatory adjustments of the diving reflex might well prevent any increase in venous return due to the hypercapnia and acidosis of apneic diving.