The study was undertaken to determine the changes in plasma catecholamine levels in response to the combined stresses of cool water immersion and hyperbaric exposure. Plasma catecholamines were measured in seven thermally unprotected trained male U.S. Navy divers immersed in water at 25 degrees C and 35 degrees C at 1 ATA and 4 ATA. All measurements were made prior to any decompression procedures. Plasma norepinephrine (NE) levels were higher during cool immersions at both 1 ATA and 4 ATA. Hyperbaric exposure during warm immersion was associated with a small but significant increase in plasma NE levels. Hyperbaric exposure during cool immersion was associated with an increase in plasma NE levels, but this increase was not statistically significant. Hyperbaric exposure in both the cool and warm immersions was associated with a moderate degree of hypoventilation and carbon dioxide retention. Plasma NE levels reflect the overall stress imposed on an individual. This study indicates that plasma NE levels may be too variable to be useful as indicators of specific stress.