Trimetaphan nitroprusside (TNP) is a new potent hypotensive agent developed to induce and maintain decreases in arterial pressure unaccompanied by resistance. This study investigated its properties and compared them with those of sodium nitroprusside (SNP) in anaesthetized greyhounds. The mean dose response to TNP was obtained by measuring haemodynamic changes in five dogs. With increasing doses, stepwise decreases in mean arterial pressure and progressive increases in heart rate occurred: cardiac index did not change significantly. In a further six greyhounds, SNP and TNP were alternately infused to induce and maintain a 30% reduction in arterial pressure for 30 min. Both drugs were short-acting, decreased systemic vascular resistance and caused tachycardia. Infusion of TNP produced lower plasma and red cell cyanide concentrations; SNP maintained hypotension with significantly less tachycardia. We conclude that there was no outstanding advantage of TNP over SNP when given as a short-term infusion in greyhounds.