Hypotensive anesthesia induced by trimethaphan was maintained at 70-79 mmHg of the systolic radial artery pressure for two hours in 35 patients undergoing spherical acetabul osteotomy. The creatinine clearance, free water clearance, fractional sodium excretion, urine volume, urinary excretion of NAG and levels of gamma-GTP, serum GOT and GPT were measured perioperatively. The creatinine clearance, free water clearance and urine volume decreased by 50% during surgery but recovered to normal values on the third day after surgery. The fractional sodium excretion remained unchanged during and after surgery. The urinary NAG activity increased from 3 +/- a (SD) to 10 +/- 5 U.g-1 and gamma-GTP activity increased from 23 +/- 7 to 33 +/- 13 U.g-1 during surgery, and these remained at high levels for three days after surgery. The serum GOT and GPT levels were within normal ranges for 21 postoperative days. The increase in urinary enzyme activities during and after surgery indicates that mild damage of renal tubular cells has occurred during hypotensive anesthesia.