We measured lung volumes, closing volume (CV), alveolo-arterial oxygen difference (P(A-a)O2) and steady-state diffusing lung capacity per liter ventilation (DLCO/V) in 18 men immersed up to the neck in water. The subjects were divided into 3 groups, according to relative changes in P(A-a)O2 and DLCO/V. In group 1 (n = 6), P(A-a)O2 decreased and DLCO/V increased, probably because of the hemodynamic changes induced by immersion. Their end expiratory level was above closing volume in water. In group 3 (n = 6), P(A-a)O2 increased and DLCO/V decreased, probably as a result of a decrease in ventilation in the dependent parts of the lung, considering that breathing range (ERV + VT) was less than closing volume. In group 2 (n = 6), P(A-a)O2 increased significantly and DLCO/V, only slightly. Tidal volume was only partially included in closing volume. The increase in exchange surface area was probably unable to compensate for the arterial hypoxia brought on by the decrease in ventilation in the dependent parts of the lung. The relationship between end expiratory level and closing volume, which seemed to explain the results observed during immersion, was itself a consequence of the subjects' age and body build.