Plasma catecholamines were measured by a modified single-isotope radioenzymic assay in five healthy medical students during and after a Finnish sauna bath with or without previous beta-blockade. The plasma noradrenaline rose more than twofold after 10 min in the hot room (100 degrees C) while the plasma adrenaline content exhibited smaller increases. The plasma noradrenaline concentration remained elevated or further increased after 5 min in a cool swimming pool (22 degrees C) whereas the exposure to cool water restored the plasma adrenaline concentration to resting values. Plasma noradrenaline, but not adrenaline, was still markedly elevated after a rest period of 15 min after the cool pool. Oxprenolol (40 mg) given 30 min before the sauna did not alter the plasma catecholamine pattern although it effectively lowered the heart rate before, during, and after the exposure to the heat stress.