Dynamic lung volumes and flows, slope of phase III of the single breath oxygen test (delta-N2), closing volume (CV), and transfer factor for carbon monoxide (TICO) were measured before and 1 h after an air dive in a hyperbaric chamber to a pressure of 0.49 MPa for 40 min. Six divers had a bottom time of 20 min and a rate of decompression of 50 kPa x min-1, and six divers had a bottom time of 24 min and a rate of decompression of 100 kPa x min-1. Decompression stops were 5 min at 0.16 MPa and 10 min at 0.13 MPa for both groups. As control exposure they were breathing O2 at atmospheric pressure for 40 min. The dive and control exposure were done on different days within 1 wk, in random order. Doppler ultrasound monitoring for venous gas microemboli (VGM) was done during the first hour after the dive. VGM were detectable in all six divers with the fast decompression rate and in one subject with the slow rate (P < 0.01). In the subjects having VGM there was a significant reduction in TICO of -5.9 +/- 4.4% compared with -0.5 +/- 3.4% after the control exposure (P = 0.034). In the five subjects without detectable VGM, the changes in TICO were -2.8 +/- 3.7% and 0.2 +/- 3.8%, respectively. There were no significant changes in dynamic lung volumes and flows, CV, or delta-N2. A reduction in TICO may reflect effects of VGM after dives in which the effect of O2 exposure is negligible.