This study was designed to evaluate the effects of trimethaphan-induced hypotension on renal function in healthy young patients undergoing maxillofacial surgery. Anaesthesia was induced with thiopentone and was maintained with halothane 1.5-2.0 per cent in oxygen. Each patient served as his own control, and data were analyzed using the paired t-test. Trimethaphan was infused at a rate of 45-52 microgram.kg-1.min-1 for an average hypotensive period of 53 +/- 4 (mean +/- SEM) minutes to reduce the mean arterial pressure (MAP) to 49 +/- 2 torr. Endogenous creatinine clearance, urinary Po2, sodium reabsorption rate (Tna), and serum and urine osmolalities were determined before, during and after arterial hypotension with trimethaphan. Urine flow averaged 2.9 +/- 1 ml/min during the period of hypotension. Endogenous creatinine clearance and Tna were significantly decreased (p less than 0.05) in the hypotensive period. These values returned to normal levels within one hour upon discontinuation of trimethaphan and restoration of blood pressure. We found no statistical difference in urine Po2, and serum and urine osmolalities during control, hypotensive and recovery periods. These results suggest that medullary renal tissue oxygenation, an index of tissue viability, may have remained adequate despite a significant reduction in endogenous creatinine clearance during the hypotensive period. Furthermore, it appears that the effect of trimethaphan-induced hypotension on renal function is similar to the sodium nitroprusside-induced hypotension in man which we have reported previously.