In two groups of patients undergoing induced hypotension with sodium nitroprusside or trimetaphan blood concentrations of lactate, pyruvate and standard bicarbonate did not differ significantly between the groups. In the nine patients who received trimetaphan there was a progressive, but statistically non-significant, decrease in mean lactate. Nitroprusside (15 patients) was associated with a small increase in mean lactate, but at low dosage there was a small decrease. No relationship to dose rate of nitroprusside was found with these short-term infusions. It is concluded that sodium nitroprusside can be used safely for induced hypotension at doses less than 1.5 mg kg-1 and that simple blood-gas analysis is adequate for the assessment of toxic effects when greater doses are given.