Solution of an equation for alveolar gas exchange during underwater ascent indicates that alveolar carbon dioxide tension should remain constant and normal during ascent owing to the expansion of gases in the lungs. Alveolar gas samples were obtained before and at the end of buoyant ascent from a depth of 90 ft of water. The partial pressure of alveolar carbon dioxide at the end was approximately equal to that at the beginning, as predicted, and the partial pressure of oxygen indicated that the arterial saturation would be almost normal. These findings indicate that buoyant ascents can be made from a depth of at least 90 ft without breathing and without urgency to breathe. Extrapolation of these findings indicates that during ascents from much greater depths there should be little or no urgency to breathe.