The influence of systemic blood flow (pump flow) and arterial blood pressure on renal function was studied during hypothermic cardiopulmonary bypass (CPB) in 14 male patients where the pump flow rate was varied between 1.45 and 2.20 l.min-1 m-2. Renal blood flow (RBF) was measured in the left renal vein with retrograde thermodilution technique and urinary flow and circulatory variables were measured with an on-line computer set-up. During CPB the RBF comprised 12-13% of the systemic blood flow and was positively related to systemic blood pressure (r = 0.71; P < 0.001) and pump flow rate (r = 0.69; P < 0.001). These findings indicate that the renal autoregulation was not operative during the hypothermic CPB period. According to multiple regression analysis, RBF was primarily determined by the pump flow rate and systemic blood pressure was of secondary importance. Urinary flow increased during hypothermic CPB and became closely related to blood pressure and pump flow. According to multiple regression analysis, urinary flow was primarily determined by systemic blood pressure.