The effect of hyperthermia on the tongue of hamsters was evaluated histologically and by measuring blood flow. The dorsal tongue surface of the male mature golden hamster was heated at 41 degrees C-45 degrees C (surface temperature) by 2450 MHz microwave for 60 min, and the regional blood flow was measured using the hydrogen clearance method at 6, 24, 42, and 60 min during heating. When heated at 41 degrees C (9 animals), 42 degrees C (11 animals), 43 degrees C (8 animals), 44 degrees C (8 animals), or 45 degrees C (8 animals), the temperature of the measurement at 0.5 mm deep from the tongue surface was 38.4 +/- 1.0 degrees C, 39.3 +/- 0.6 degrees C, 40.2 +/- 0.6 degrees C, 40.8 +/- 0.5 degrees C, or 43.5 +/- 1.0 degrees C (Mean +/- S.D.), respectively. In the groups heated at 41 degrees C, 42 degrees C, 43 degrees C, 44 degrees C, the blood flow was significantly increased at the first measurement (6 min) and remained elevated during heating. The blood flow was further increased during the second heating which took place after a 2 hour interval. The group heated at 45 degrees C showed a gradual decrease in the blood flow after the initial increase. The second heating induced a slight increase to a lower blood flow than the control. Histologically, dilation and congestion of venules, hemorrhage, interstitial edema in the musculature, and swelling of muscle fibers were found. Dilation, congestion, and rapture of capillaries beneath the mucosal epithelial layer were found also. These damages were more severe with an increase in heating temperature and duration. Additionally, thromboses were found in the group heated at 45 degrees C which was examined 2 hours after heating. The heated part was necrosed in every case heated at 45 degrees C. These results suggest that the critical surface temperature at which the tongue of hamsters receive irreversible damage lies between 44 degrees C and 45 degrees C.