In 12 patients with acute myocardial infarction the hemodynamic effects of a single intravenous injection of 30 mg of pentazocine were investigated. The administration of pentazocine resulted in no significant hemodynamic changes. In particular, there was no increase in peripheral vascular resistance and no evidence of decreased left ventricular function or respiratory depression. In the occasional patients who demonstrated an elevation of pulmonary arterial pressure, it was probably due to a direct effect of the drug on the pulmonary vasculature.