AIMS: Previous studies suggested haemodynamic benefits and, possibly, mortality reduction with the use of nitric oxide synthase (NOS) inhibition in patients with acute myocardial infarction (AMI) complicated by cardiogenic shock (CS). We assessed preliminary efficacy and safety of four doses of l-n-monomethyl-arginine (l-NMMA), a non-selective NOS inhibitor, in patients with AMI complicated by CS despite an open infarct-related artery. METHODS AND RESULTS: Patients (n = 79) were randomly assigned to a bolus and 5 h infusion of placebo or 0.15, 0.5, 1.0, or 1.5 mg/kg of l-NMMA. The primary outcome measure was absolute change in mean arterial pressure (MAP) at 2 h. Fifteen minutes after study drug initiation, mean change in MAP was -4.0 mmHg in the placebo group and 5.8 (P = 0.02), 4.8 (P = 0.02), 5.1 (P = 0.07), and 11.6 (P < 0.001) mmHg in the four l-NMMA groups, respectively (all vs. placebo). Mean change in MAP at 2 h was -0.4, 4.4, 1.8, -4.1, and 6.8 mmHg in the placebo and four l-NMMA groups, respectively (all P = NS). CONCLUSION: l-NMMA resulted in modest increases in MAP at 15 min compared with placebo but there were no differences at 2 h.