Tolazoline was infused intravenously (2 mg/kg over 2 minutes) in awake neonatal and mature calves. In normoxic pulmonary normotensive neonatal calves, tolazoline induced minimal hemodynamic changes, except for an immediate bradycardia. Tolazoline caused increases in systemic arterial pressure and vascular resistance and a bradycardia in normoxic normotensive mature calves. Calves were also made hypoxic, by breathing 12% O2 by facemask, in order to produce pulmonary hypertension. In hypoxic pulmonary hypertensive neonatal calves, tolazoline induced transient pulmonary vasodilation, a reduction in systemic arterial pressure, and marked bradycardia. Similarly, tolazoline caused reductions in pulmonary and systemic arterial pressure and bradycardia in hypoxic pulmonary hypertensive mature calves. Pentobarbital anesthesia in neonatal calves induced marked hemodynamic changes but did not alter the cardiovascular responses to tolazoline. A significant correlation was found between the baseline pulmonary arterial pressure and the magnitude of the pulmonary depressor response to tolazoline when all interventions were evaluated. Thus, tolazoline induced transient pulmonary vasodilation in pulmonary hypertensive calves but simultaneously caused profound transient bradycardia, particularly in neonatal calves.