Hemodynamic and respiratory variables were measured in 18 supine patients with acute myocardial infarction uncomplicated by left ventricular failure. Measurements were done during a control period and then when pulmonary capillary bed filling was decreased by iv injection of 3 mg nitroglycerin (NTG) and finally when pulmonary capillary bed filling was increased by raising the patients' legs. Heart rate increased significantly (p less than .05) ventricular filling, and mean aortic pressures dropped significantly (p less than .001) after NTG injection. Heart rate and mean aortic pressure returned to control level and ventricular filling pressures increased significantly (p less than .001) after elevating patients' legs. Mean carbon monoxide diffusion capacity decreased significantly (p less than .01) from 9.8 +/- 3.4 to 8.5 +/- 2.9 ml/min X mm Hg after NTG injection and increased significantly (p less than .02) to 9.3 +/- 3.5 ml/min X mm Hg after elevating patients' legs. Pulmonary wedge pressure decreased significantly (p less than .001) from 15 +/- 5 to 8 +/- 4 mm Hg after NTG injection and increased significantly (p less than .001) to 10 +/- 4 mm Hg after elevating patients' legs. We conclude that NTG reduces pulmonary capillary filling and pulmonary diffusion by reducing the available pulmonary gas exchange surface.