Plasma lipids, blood glucose, plasma insulin (IRI) and serum dopamine-beta-hydroxylase (DBH) were measured in 30 subjects undergoing surface-induced deep hypothermia with circulatory arrest for open-heart surgery. Non-esterified fatty acid (NEFA) in the plasma rapidly increased at the lowest temperature (23 degrees C) reached and other lipids in the plasma decreased during the cooling period. An increase of NEFA and a decrease of triglyceride have been attributed to the action of lipoprotein lipase activity stimulated by heparin. It is also likely that the decrease of other lipids and beta-lipoprotein in the plasma results from the transient hypofunction of the liver due to hypothermia. Blood glucose increased during the cooling period, while plasma insulin showed no significant change. Serum DBH reflecting catecholamine also showed no significant change during the cooling or rewarming periods. Therefore, hyperglycemia in hypothermic open-heart surgery may result from the decrease of peripheral utilization of glucose and from the inhibition of insulin secretion due to the transient pancreatic hypofunction.