Although studies in human beings have suggested that sodium nitroprusside may increase the diastolic compliance of the left ventricle, animal models have provided contradictory evidence following pericardiotomy. The effect of nitroprusside on the human left ventricle after pericardiotomy has not been reported. Accordingly, we studied the effects of intravenous sodium nitroprusside in eight patients during cardiac operations. Curves relating left ventricular end-diastolic pressure (EDP) to echocardiographically determined end-diastolic diameter (EDD) were determined during the transfusion of volume from the reservoir of the heart-lung machine (EDP varied from 5 to 35 mm Hg) before and during nitroprusside administration (1 to 4 mcg/kg/min). An average relation (EDP = 0.41 +/- 0.40 [SD] e1.18 +/- 0.58 EDD was obtained for eight patients by exponential curve fitting (r = 0.80 +/-08). Nitroprusside decreased systolic left ventricular pressure (118 +/- 16 to 100 +/- 13 mm Hg, p less than 0.001, paired t test) but did not alter the control curve relating EDP to EDD. We conclude that EDD-EDP relations of the human left ventricle are not altered by nitroprusside administration in the absence of the restraining influence of the pericardium. This suggests that nitroprusside has no significant direct effect on diastolic properties of the human myocardium.