Four balloon-guided pulmonary artery catheters from each of two manufacturers were placed in eight anesthetized dogs, and balloon volumes were measured at randomized time intervals from 1 to 30 min during (1) ventilation with 100% oxygen and (2) ventilation with 70% nitrous oxide in oxygen. Catheters from one manufacturer showed no increase in balloon volume in either condition. The balloon volumes of catheters from the second manufacturer increased up to 17% during ventilation with nitrous oxide. Balloons of these catheters required less inflation pressure than those from the first manufacturer. It was concluded that the effects of nitrous oxide on the balloon volumes of pulmonary artery catheters in vivo are of little clinical significance.