The cardiovascular effects of i.v. cimetidine 200 mg or ranitidine 50 mg were investigated in a double-blind cross-over study of 20 critically ill patients in an intensive therapy unit. Both agents caused transient decreases in mean arterial pressure and systemic vascular resistance, although there was no significant change in heart rate, cardiac output, central venous pressure, pulmonary artery wedge pressure or pulmonary vascular resistance in either group. The hypotensive, systemic vasodilatory effect was more common after cimetidine than after ranitidine. Also, it was significantly greater after cimetidine. Both histamine H2-receptor antagonists may cause hypotension in the critically ill patient.