Lung function and the response to exercise were monitored in seven diver/welders who took part in a test saturation dive to 300 m for an average duration of 12 days; decompression took an average of nine days. Immediately after the dive the forced vital capacity was increased above base line by on average 0.51, the forced expiratory volume by 0.281 and peak expiratory flow rate by 0.71 s-1. There was no change in flow rate at small lung volumes (FEF 75% FVC). Recovery was complete and appeared to have a half time of 28 days. Transfer factor of the lungs for carbon monoxide (TlCO) was reduced by on average 9.6% after the dive but while partial recovery occurred, the values at one year were on average lower than those observed initially. The reason is unclear. One subject developed transient oxygen toxicity with stiff lungs and increased ventilation and cardiac frequency during submaximal exercise; a second subject developed similar changes but without accompanying symptoms. There is need for detailed physiological surveillance of people undertaking deep dives; this should be undertaken in circumstances that permit accurate measurements and full subject cooperation.