Intravenous cimetidine 200 mg, and ranitidine 50 mg were administered as 2 minute infusions to 50 intensive-care patients. Cimetidine and ranitidine differed significantly in their effect on systolic arterial blood pressure measured during the second minute (alpha = 0.01). In the case of cimetidine the haemodynamic parameters measured over 10 minutes revealed a clearly defined fall in systolic, diastolic and mean arterial blood pressure as well as a rise in heart rate. The initial values were regained 5 minutes after drug application. Adverse effects of ranitidine on haemodynamics were much less than those of cimetidine. Neither drug produced any essential change in clotting parameters (partial thromboplastin time, plasma thrombin time, thromboplastin time and fibrinogen) measured before and 5 minutes after application, nor had they any effect on blood-gases or acid-base status.