The cardiovascular and acid-base changes following equivalent i.v. bolus doses of sodium nitroprusside (SNP) and potassium cyanide (KCN) have been studied in two groups of anaesthetized dogs. In a third group, the metabolic changes produced by i.v. infusion of SNP 1.5 mg kg-1 at a constant rate over 1 h have been studied. In contrast to a decrease in arterial pressure following SNP, hypertension and tachycardia occurred after the administration of KCN, with hyperventilation and an increase in packed cell volume. During infusion of SNP, increases in plasma cyanide concentrations were associated with an increase in arterial base deficit, plasma lactate and excess lactate and a decrease in oxygen consumption. The occurrence of lactic acidosis with SNP 1.5 mg kg-1 suggests that this may be the maximum safe dose for short term infusion. However, all these changes reversed spontaneously following discontinuation of SNP, indicating that base deficit is an adequate metabolic monitor during administration of SNP.