1. Plasma renin activity, plasma adrenaline and noradrenaline concentrations, and urinary adrenaline and noradrenaline excretion rates were measured in ten subjects during 3 days of exposure to a simulated altitude of 12 000 ft.
2. In both the supine and standing positions, renin activities were suppressed during all 3 days at altitude.
3. Plasma noradrenaline and adrenaline concentrations were significantly increased by the third day at altitude.
4. Urinary adrenaline excretion tended to be increased during the entire 3 days at altitude, with no significant change between the first and third day. Noradrenaline excretion was significantly increased on the third day.
5. The finding of decreased renin levels suggests that the enhanced activity of the sympathetic nervous system at high altitude does not stimulate renin release.