The coronary resistance, total heat production, oxygen consumption and isovolumic mechanical performance were measured simultaneously in the isolated beating rabbit heart. The direct effects of nitroglycerin (0.12 mg/l) were determined. The expected coronary dilatation occurred; coronary resistance fell by 21.5% independent of mechanical performance. There was a 14% fall in mechanical performance at the apex of the Frank-Starling curve and lesser reductions at less-than-apical diastolic volumes. Oxidative metabolism was not affected; there was no change in the calorific equivalent of oxygen (20.97 mJ/mul:O2) nor was there any significant anerobic metabolism. The ergonic cost of the force-independent metabolism which includes calcium activation energy was reduced by 17.7%. The unit metabolic cost of mechanical performance, or the performance-dependent metabolism, was unchanged resulting in a net increase in overall mechanical economy. It is concluded that it is against a background of a more metabolically efficient but less mechanically active myocardium that nitroglycerin exhibits its well known peripheral and reflex in the intact organism.