Plasma growth hormone (plasma GH) and blood-glucose concentrations were measured in 23 patients undergoing open heart surgery with moderate hypothermia. A significant increase in blood-glucose concentration occurred with sternotomy and increased during bypass, partly as a result of the exogenous glucose load from the perfusate. Following bypass, the blood-glucose remained above the pre-anaesthetic concentration, and this elevation persisted into the period following surgery. Plasma GH also increased with surgery and remained elevated during perfusion. The highest concentrations occurred following bypass when normal temperature had been regained.