Sodium nitroprusside (SNP) was used to produce deliberate hypotension in 30 selected patients, 9 to 78 years of age, for total hip replacement under halothane-N2O-O2 anesthesia. Hypotension was induced in the first 13 patients by infusing a 0.01% (100 mug/ml) solution of nitroprusside (NP) in 5% dextrose. Blood pressure was diminished to a level just producing a dry surgical field. Preliminary data demonstrated that the mean arterial blood pressure (MAP) achieving this condition was 65 torr (p less than 0.01) and that the minute dosage of NP (mug/min) required to consistently reduce MAP to 65 torr could not be predicted on the basis of body weight. However, the age/weight ratio (yr/kg) of each patient, plotted against the known minute dosage of NP given during anesthesia, produced a highly significant dose-response curve (p less than 0.001, r = -0.8226). The preliminary dose-response curve was examined in a double-blind study on an additional 17 patients. The curve derived from the prospective study did not differ from that of the preliminary study. In addition, the combined data from the retrospective and prospective studies (30 patients) gave a better statistical fit than did those from the preliminary study alone (p less than 0.001, r = -0.8939). The nomogram provides an additional margin of safety in the use of this potent, fast-acting drug. SNP has been found predictable and effective in reducing surgical blood loss in selected patients undergoing total hip replacement.