Thirteen healthy males were repeatedly studied during the exposure to a moderate hyperbaric environment of 4--6, 7 and 9 ATA, in a hyperbaric chamber and partly during immersion in sea water (3.5--4 ATA). The maximal duration of the exposure was 35 min and that of decompression 90 min. As compared to the dynamics of the change found in controls, under hyperbaric conditions a significant prolongation of the visual reaction time (especially during sea dives), and an impairment of the mental performance in an arithemetical calculation test were found. There occurs an increase of the excretion in urine of cortisol, epinephrine and vanillylmandelic acid, especially after immersion in sea.