Peroperative measurements of renal blood flow (RBF), renal O2-uptake, and renal venous lactate/pyruvate (L/P) ratio were performed before and after a period of 30-71 min of hypothermic (10-15 degrees C) renal ischaemia in nine patients, undergoing surgery for renal calculi. Before ischaemia, RBF correlated inversely to arterial-renal venous O2-difference (r = -0.74, P less than 0.05, n = 9) and directly to the preoperatively estimated unilateral glomerular filtration rate (r = 0.76, P less than 0.05, n = 8). After hypothermic ischaemia RBF decreased on the average by 42% (P less than 0.01) immediately after re-established perfusion and 36% (P less than 0.02) 30 min later. In one additional patient, who had a short warm ischaemia (8 min), the flow pattern was the same. As arterial pressure remained constant, the reduced RBF signifies an increased renal vascular resistance. Renal O2-uptake and renal venous L/P ratio were almost constant, indicating no significant anaerobic processes being involved in the flow response. None of the patients showed any signs of reactive hyperaemia. It is concluded that hypothermic renal ischaemia may be followed by an increased renal vascular resistance even when signs of anaerobic metabolism are sparse or absent. This conception is in contrast to that gained from previous animal experiments where anaerobic processes are kept responsible for the changes. The mechanism behind the described response is unknown, but the changes are probably reversible.