We investigated the effect of coronary artery bypass grafting on the rate of left ventricular relaxation as defined by the time constant for isovolumetric relaxation, T, measured in milliseconds. Completeness of relaxation at rapid heart rates was determined by comparison of the relationship between left ventricular pressure and echocardiographic left ventricular cross-sectional cavity area during rapid ventricular pacing with that obtained after a prolonged diastole when the ventricle was maximally relaxed. Twelve patients with coronary artery disease had significantly higher T values (94.5 +/- 6.2) than six patients without coronary artery disease who were undergoing other open heart procedures (39.5 +/- 5.0, p less than .001). T was significantly reduced after coronary artery bypass grafting (68.2 +/- 5.1, p = .007), but was unchanged in the six control patients after cardiopulmonary bypass (37.8 +/- 4.5, p = .54). Similar changes were found during rapid pacing to 100, 120, and 140/min. Incomplete relaxation was detected in three of 10 (heart rate 120 beats/min) and nine of 11 (heart rate 140 beats/min) patients with coronary artery disease and this decreased to 0 of 10 (heart rate 120 beats/min) and six of 11 (heart rate 140 beats/min) patients after coronary artery bypass. Incomplete relaxation before bypass at a heart rate of 120 beats/min averaged 0.9 +/- 0.3 mm Hg. At a heart rate of 140 beats/min, incomplete relaxation averaged 5.6 +/- 1.6 mm Hg before and 1.4 +/- 0.5 mm Hg after bypass. Intake of beta-blockers or calcium-channel blockers, body temperature, and systolic blood pressure were not found to be related to these changes. We conclude that immediately after coronary artery bypass relaxation of left ventricular muscle is enhanced and incomplete relaxation at rapid heart rates is less likely. The most probable cause of this improvement in ventricular relaxation after coronary artery bypass grafting is relief of ischemia.