The authors studied 12 patients who required deliberate hypotension for spinal fusion operations in order to investigate the efficacy of captopril for reducing dose requirement for sodium nitroprusside (SNP). Six patients, selected at random, were pretreated with captopril, 3 mg/kg po, and the remaining six patients served as controls. All patients received a similar anesthetic technique, consisting of thiopental 3 mg/kg, pancuronium 0.1 mg/kg, morphine 0.5 mg/kg, plus nitrous oxide 70% in oxygen. SNP was used to maintain mean arterial pressure (MAP) at 50–55 mmHg during deliberate hypotension lasting 140 · 13 minutes (mean · SE). Patients who received captopril required less SNP than untreated patients both early during hypotension (1.4 · 0.5 μg · kg−1· min−1vs.4.8 · 0.8 μg·kg−1· min−1,P< 0.05), as well as late during hypotension (2.2 · 0.2 μg · kg−1· min−1vs.5.6 · 0.6 μg · kg−1· min−1,P< 0.05). Whole blood cyanide was significantly lower in the patients pretreated with captopril than the untreated controls both early in the hypotensive period (2.7 · 0.6 μmol/1vs.13 · 4 μmol/1,P< 0.05) and also late in the hypotensive period (3.7 · 0.8 μmol/1vs.30 · 10 μmol/1,P< 0.05). MAP was reduced by captopril pre-treatment both following induction of anesthesia (64 · 4 mmHg captoprilvs.80 · 4 mmHg control,P< 0.05) and during surgery before deliberate hypotension (86 · 5 mmHg captoprilvs.100 · 4 control,P< 0.05). Cardiac output did not differ significantly between the groups, either awake or after anesthetic induction. The authors conclude that captopril pretreatment significantly reduces the dose of SNP required to produce deliberate hypotension and, therefore, reduces the potential for cyanide toxicity. No adverse hemodynamic consequences of combining captopril with thiopental, N2O, or morphine anesthesia were observed.