Ranitidine, a furan H2 receptor antagonist, is increasingly prescribed in the ICU setting. Before the availability of parenteral ranitidine, cimetidine had been the sole H2 blocker used to decrease gastric acidity in this setting. However, cimetidine has recently been shown to transiently, but significantly, reduce mean arterial pressure due to direct vasodilation in a critically ill population. Therefore, this double-blind prospective study was designed to examine the hemodynamic effects of iv ranitidine in an ICU population. Twenty-six critically ill hemodynamically stable patients were studied; all patients were already receiving ranitidine and had an arterial line and a pulmonary artery catheter in place. This study demonstrated that an iv infusion of ranitidine did not produce clinically significant hemodynamic effects in this patient population.