Plasma renin activity (PRA) and aldosterone concentration (PA) increased in eight men following a brief (30--40 min) heat exposure (50 degrees C dry bulb, 25 degrees C wet bulb) with light work. Sweat loss was less than 1% body weight. Plasma cortisol concentration was unchanged or decreased. In four subjects, a standard heat test was repeated in winter and summer (natural acclimatization). The increase in PRA and PA following heat exposure was less in summer than in winter. Four other subjects were artificially acclimated by daily work periods of 90 min at 50 degrees C for 7 days (artificial acclimation). Heat-induced elevation in PRA was considerably reduced by artificial acclimation, although postheat PA was reduced in only two of the four subjects. The small degree of sweat loss under the conditions of these experiments shows that circulating renin and aldosterone levels are increased in the heat even when a significant sodium deficit is not incurred.