The relative influences of the supine posture and of immersion on the renin-aldosterone system (RAS) were studied at rest and during moderate exercise in five healthy men. When supine, resting or immersion to the neck for 20 min in a thermoneutral environment both induced a decrease in plasma renin activity (PRA) when compared with the levels measured after 15 min sitting at rest (resting: -44%, p less than 0.05. Immersion: -45%, p less than 0.05). There was no significant difference in PRA decrease between the two situations. Aldosterone (ALDO) values were lower after supine rest or immersion than those observed after sitting at rest, but the difference was not significant. Two types of exercise at a constant relative work load (40-50% maximal oxygen uptake), namely cycling on an ergocycle in the supine position and free-style swimming, induced increases in PRA and ALDO when compared with the levels measured after 15 min rest when sitting (respectively, PRA = +35%, p less than 0.05, and +45%, p less than 0.05, ALDO = +32%, p less than 0.01 and +35%, p less than 0.05). Increases in PRA and ALDO did not differ between the two exercises. Thus inhibitory effects on RAS of change in external pressure are negligible during water immersion to the neck in the supine position and during swimming at moderate intensity.