Following partial coronary occlusion, the comparative effects of administration of nitroglycerin and nitroprusside on aortic perfusion pressure, coronary flow, local tendon, and segmental length were assessed. Partial coronary occlusion decreased coronary flow from 78.9 +/- 5.0 ml/gm to 40.6 +/- 2.6 ml/100 gm and total tension to 76.9 +/- 5.6 percent of the control value. Nitroglycerin and nitroprusside improved total tension to a similar degree with a reduction of either 10 or 20 mm Hg in systolic pressure; however, when systolic pressure was reduced by 30 mm Hg, nitroglycerin decreased total tension slightly but insignificantly, while nitroprusside significantly decreased ischemic zone tension. Changes in segment length shortening paralleled changes in total tension. Thus, in the presence of partial coronary occlusion, both nitroglycerin and nitroprusside similarly affect coronary blood flow, total tension, and systolic shortening at a decrease of 10 and 20 mm Hg in systolic pressure; however, at a decrease of 30 mm Hg, both nitroglycerin and nitroprusside decrease total tension and systolic shortening.