We have examined the effect of profound hypothermia on gut mucosal perfusion in 20 infants, aged 1.4-45 weeks, requiring cardiopulmonary bypass (CPB). After induction of anaesthesia, a laser Doppler probe was inserted 8 cm into the patient's rectum to allow monitoring of rectal mucosal perfusion ("flux") throughout operation. Steady-state observation periods (5 min with no change in temperature or mean arterial pressure (MAP) were achieved after 10 min on CPB at 35 degrees C, after CPB-induced cooling to 15-25 degrees C, immediately before rewarming and after rewarming to 35 degrees C. Throughout these periods flow rate was 100 ml kg-1 min-1, packed cell volume was kept constant and Paco2 maintained at 5.3 +/- 0.5 kPa. No vasoactive drugs were used. Initial warm and rewarm MAP values (46 mm Hg) were significantly lower (P = 0.008) than during the cold CPB periods (63 and 64 mm Hg). Mean flux in the first cold period (152) was significantly lower (P = 0.001) than that in the first warm CPB period (211). Post-rewarm flux (127) was significantly lower than all other CPB flux values (P = 0.004). We conclude that although hypothermia significantly reduced mucosal blood flow, rewarming produced even greater reductions in mucosal perfusion that may prove crucial in the development of mucosal hypoxia.