The cardiovascular effects of hyperbaric (3 atm abs) air, 100% oxygen, and hyperbaric oxygen (HBO2) at 3 atm abs were investigated in 13 pentobarbital-anesthetized dogs. We measured heart rate, arterial pressure, pulmonary artery pressure, right atrial pressure, left and right ventricular pressure, and cardiac output. From these we determined end diastolic pressure, +/-maximal dp/dt, maximal (dp/p), end systolic elastance, cardiac work, and systemic (SVR) and pulmonary vascular resistance (PVR). Pulmonary blood volume was obtained from the mean transit time of ascorbic acid. The significant results with HBO2 were a decrease in heart rate, cardiac output, and cardiac work. All left ventricular performance indices decreased, without a change in preload or afterload. In contrast, only right ventricular -dp/dt decreased. SVR increased but PVR did not change; 100% O2 produced similar but less pronounced responses. Hyperbaric air had only mild effects. Pulmonary blood volume and lung wet/dry ratio did not change. Our data suggest that HBO2 may act by a differential effect on the autonomic innervation of the right and left ventricles. The resultant ventricular imbalance may be of clinical importance in the mechanism of pulmonary edema in patients in congestive heart failure undergoing hyperbaric therapy.