Pathophysiological effects of exposure to Finnish sauna (80-90.degree. C, 30-40% relative humidity) were investigated in 60 volunteers, 33 men and 27 women aged between 18-63 yr. The volunteers entered the sauna after a rest period of 20 min, and remained there for 20 min or until they suffered discomfort. Weight, height, rectal and skin temperatures, blood pressure, heart and respiratory rates and ECG were recorded 20 min prior to the sauna, during the sauna and 20 min after leaving the sauna. Marked physiological changes appeared in the first few minutes in the sauna without any prodromal warning. At the 20th min the mean heart rate was 143 .+-. 25 (SD) beats/min (> 160 beats/min in 32% of the subjects). Mean rectal temperature was 38.6.degree. .+-. 0.6.degree. C (SD) (> 39.degree. C in 22%); mean skin temperature was 40.4.degree. .+-. 1.degree. C (SD) (> 40.degree. C in 35%); mean systolic blood pressure was 130.5 .+-. 26.6 (SD) mm Hg (> 160 mm Hg in 17%); and mean diastolic pressure was 66.6 .+-. 15.9 (SD) mm Hg (< 50 mm Hg in 17%). The mean total sweat loss was 457g .+-. 264g (SD). Subjects (3) experienced syncope, and 1 developed an anginal attack; ECG changes suggestive of coronary insufficiency were recorded. No decrease in blood pressure occurred in patients with preexisting high blood pressure. Sauna bathing involves dangers to the bather''s health, which may appear suddenly, without prodromal warning signs.