To determine if denitrogenation mitigates the cardiovascular effects of venous air emboli, air was infused intravenously in halothane-anesthetized dogs breathing oxygen (02) or 75 to 79 percent nitrogen (N2) (02-supplemented air). During a constant infusion rate of 3.5 or 7 ml./min. in 6 dogs, the volume of air required to produce a 40 percent decrease in mean arterial pressure (MAP) with 02. breathing was double the volume required during air breathing. In 4 additional dogs, 3 ml./kg. of air injected over 4 to 5 minutes decreased MAP 35 to 40 percent, but breathing air or 02 produced no difference in the spontaneous rate of return of MAP to 90 percent of control. The results indicate that when air slowly enters the venous circulation, larger volumes are necessary to produce cardiovascular depression in animals breathing 02. However, there is no significant difference in the rate of cardiovascular recovery during air versus 02 breathing once depression occurs.