The hypothesis was tested that facial sweating at the end of cardiopulmonary bypass (CPB) is a thermoregulatory phenomenon. Twenty-two patients undergoing cardiac surgery with fentanyl anesthesia were studied. Nasopharyngeal temperature, nasal skin temperature, rectal temperature, and mean skin temperature were monitored for 90 minutes after the start of rewarming on CPB. Calf-toe and forehead-nose skin temperature gradients were followed as a measure of peripheral and facial thermoregulatory vasoactive responses. Facial sweating was defined as grade 1 (noticeable) or grade 2 (obvious droplets). Fourteen patients (64%) sweated during rewarming at the end of CPB. In 11 cases the onset of sweating was preceded by a dramatic increase in nasal skin temperature (mean +/- SEM, 4.6 +/- 0.3 degrees C in 5 min), suggesting facial vasodilation. The maximum rate of increase (degree C/5 min) in nasal skin temperature was significantly greater in patients who sweated than in those who did not, 4.1 +/- 0.4 degrees C versus 2.6 +/- 0.3 degrees C (P < 0.015). There was no difference in the age, weight, or BSA between patients who sweated during CPB and those who did not. The nasopharyngeal temperature threshold for the onset of sweating was not elevated (grade 1, 36.4 +/- 0.5 degrees C; grade 2, 37.6 +/- 0.4 degrees C), but there was a 5 to 6 degrees C interpatient variation. It was concluded that facial sweating during rewarming on CPB is typical of a thermoregulatory response. Absence of sweating in one third of patients may be due to pharmacokinetic or pharmacodynamic differences in the response to anesthesia.(ABSTRACT TRUNCATED AT 250 WORDS)