The effects of the high temperature (80-120 degrees C) of the Finnish Sauna bath on the concentrations of growth hormone, immunoreactive insulin and renin activity in plasma, on blood glucose and on the urinary excretion of aldosterone, vanilmandelic acid and sodium of 55 healthy volunteers were studied. There was a significant increase in mean heart rate (62%), serum growth hormone (142%) and plasma renin activity (95%) in the Sauna. One hour after the Sauna bath the mean serum growth hormone had returned to the control level while plasma renin activity still remained higher (p less than 0.05) than before the Sauna bath. The serum insulin, blood sugar and urinary excretion of aldosterone and VMA did not change during or after Sauna bath. The urinary sodium excretion decreased significantly after the Sauna bath and the decrease was most striking (46%) during the first 6-hour period from the beginning of Sauna bath. Plasma renin activity values correlated positively with 12-hour urinary VMA excretion (p less than 0.01) and negatively with 6-hour urinary sodium excretion (p less than 0.05) before and after Sauna, suggesting the role of catecholamines and sodium depletion in renin response in Sauna.