The determinations carried out on 12 divers during the immersion into the sea to a depth between 7 and 29 m, as well as on controls in immersions at 4 m revealed a significant prolongation of the visual reaction time and a marked tachycardia. The changes were more marked at great depths and if exercise was superimposed over hyperbaric stress. A rise in the urinary excretion of 17-HOCS and vanillylmandelic acid and an impairment of the manual dexterity were also found during immersions. The results stress the importance of divers' monitoring during their underwater activity and the necessity of improving their physical training.