A double blind investigation of the hypotensive effect of morphine in patients with uncomplicated myocardial infarction has been performed in 20 patients. Ten were given an intramuscular injection of 1.5 cg morphine sulphate and ten an injection of a placebo. Blood pressure was recorded with the patient lying flat in bed, bead up, in an armchair position. A slight tendency to development of orthostatic hypotension was observed after the injection of morphine. Cardiac output was determined in five patients with uncomplicated myocardial infarction after the i.v. injection of 0.75 cg morphine sulphate. No marked fall in cardiac output was observed.