Before and after saturation dives, we measured lung volumes and diffusing capacity (DL(CO)/V(A] with a single-breath method on 12 divers (6 divers per dive) who participated in 300-msw saturation dives (total dive time of 15 days with P(O)2 = 0.42 atm abs) and on 6 divers who were engaged in a 320-msw saturation dive (6 days of bottom time with P(O)2 = 0.42 atm abs and 12 days of total decompression time with P(O)2 = 0.495 atm abs. In all divers, vital capacity right after surfacing did not significantly decrease compared with predive values. In the 300-msw saturation divers after surfacing, DL(CO)/V(A) adjusted for hemoglobin (Hb) changes did not significantly decrease, but in the 320-msw saturation divers Hb-adjusted DL(CO)/V(A) was significantly (P less than 0.001) decreased by 0.70 (mean) +/- 0.21 (SD) (ml.min(-1).mmHg(-1).liter(-1]. These observations indicate that the diffusing capacity is a more sensitive index of pulmonary oxygen toxicity than vital capacity that is traditionally used, and that oxygen partial pressure less than 0.5 atm abs can induce a decrease in pulmonary function.