To compare the cardiovascular effects of the histamine H2 receptor antagonists, famotidine and cimetidine, in critically ill patients, seven ICU patients were given 20 mg of famotidine or 200 mg of cimetidine intravenously in a randomized fashion. Each patient was studied on 2 separate days. In a random fashion, they received famotidine on one of the days and cimetidine on the other. Mean systemic arterial pressure (MAP) was maximally decreased 2 min after intravenous cimetidine from 103 +/- 10 (13.7 +/- 1.3 kPa) (mean +/- s.d.) to 83 +/- 15 mmHg (11.1 +/- 2.0 kPa) (P less than 0.05). MAP returned to control values 8 min after administration of the drug. Systemic vascular resistance (SVR) was significantly decreased during the 8-min observation period (P less than 0.01). In contrast, famotidine produced little haemodynamic effect over the 8-min period. Therefore, we suggest that famotidine may be a better H2 antagonist than cimetidine in critically ill patients requiring vasoconstrictor drug support, since it avoids the significant decrease in systemic blood pressure and peripheral vasodilation seen after cimetidine.