Cyanide antidotes were given to dogs before an infusion of sodium nitroprusside 1.5 mg kg-1 for 1 h. Dogs given thiosulphate 75 mg kg-1 had significantly lower plasma and red cell cyanide concentrations while plasma thiocyanate concentrations were significantly increased in comparison with control. These changes were associated with only minimal disturbance of tissue oxygenation. There was no effect on red cell cyanide or thiocyanate concentrations in dogs treated with hydroxocobalamin 1.5 mg kg-1, but plasma cyanide concentrations were significantly greater than in those receiving no antidote although there was less evidence of impaired oxygenation. There was no evidence of a synergistic action between thiosulphate and hydroxocobalamin. The vascular response to nitroprusside was unchanged in the thiosulphate-treated dogs, but was significantly greater in those given hydroxocobalamin. The implications for prophylaxis and treatment of cyanide poisoning following nitroprusside overdose are discussed.