The cardiovascular effects of ranitidine were studied in 12 children with congenital heart disease who had been given tolazoline as a pulmonary vasodilator. Ranitidine was given as prophylaxis against gastrointestinal haemorrhage induced by tolazoline. Tolazoline 1-2 mg/kg caused significant falls in pulmonary and systemic vascular resistances and a rise in heart rate. After intravenous administration of ranitidine 3 mg/kg both resistances rose again and neither resistance then differed significantly from baseline levels. Heart rate also fell and the final heart rate was significantly below baseline levels. We conclude that there may be H2 receptors within the pulmonary and systemic circulations and that tolazoline may mediate some of its effects through these H2 receptors rather than by alpha adrenergic receptor blockade. The safety of H2 blockade in children, particularly those with pulmonary hypertension, needs further investigation.