It is believed that nitroglycerin causes vasodilatation by interacting with sulfhydryl groups present in vascular smooth muscle. This study was performed to assess whether methionine, an amino acid capable of increasing sulfhydryl availability, would potentiate the hemodynamic effects of nitroglycerin. Nitroglycerin was initially infused in incremental doses from 1 to 50 micrograms/min in all patients to determine the dose required to reduce mean arterial pressure by 10% and pulmonary capillary wedge pressure by 30%. After a washout period, 10 patients received 5 g methionine i.v. and five patients received an equal volume of 5% dextrose in water (control). Nitroglycerin dose titration was then repeated. Methionine alone induced no hemodynamic effects, but after methionine infusion, there was a significant reduction in the nitroglycerin infusion rate required to reach each hemodynamic endpoint (p less than 0.01). In the control group, there was no significant change in responsiveness to nitroglycerin between infusions. Thus, methionine potentiates the hemodynamic effects of acutely administered intravenous nitroglycerin.