Pressure and dimension analyses were used to quantitate the changing cardiac response to dopamine over a 24-hour interval after coronary artery bypass grafting (CABG). Ultrasonic dimension transducers were utilized to measure the minor-axis diameter of the left ventricle, and matched micro-manometers were inserted to measure intracavitary left ventricular pressure and intrathoracic pressure. Pressure and dimension data were recorded and analyzed by computer during dopamine infusion at 0, 2.5, 5.0, and 10.0 μg per kilogram per minute, at periods designated as early (2 to 4 hours after CABG) and late (18 to 24 hours after CABG). Myocardial contractile responses to dopamine (peak velocity of minor-axis shortening, maximal excursion) were similar at each dose in the early and late studies. However, overall hydraulic performance, as reflected by cardiac outputs and the areas of the pressure/diameter work loops, had augmented late dose responses. This study suggests a major change in the relationship between the heart and peripheral control mechanisms that may partially explain diminishing inotropic requirements over time, in addition to the generally accepted occurrence of improvement in contractile state and functional reserve following cardiac operation.