To determine the role of preload in maintaining the enhanced stroke volume of upright exercise-trained endurance athletes after deconditioning, six highly trained subjects undergoing upright and supine bicycle ergometry were characterized before and after 3, 8 and 12 weeks of inactivity that reduced oxygen uptake by 20%. During exercise, oxygen uptake, cardiac output by carbon dioxide rebreathing, cardiac dimensions by M-mode echocardiography, indirect arterial blood pressure and heart rate were studied simultaneously. Two months of inactivity resulted in a reduction in stroke volume, calculated as cardiac output/heart rate, during upright exercise (p < 0.005) without a significant change during supine exercise. A concomitant decrease in the left ventricular end-diastolic dimension from the trained to the deconditioned state was observed in the upright posture (5.1 ± 0.3 versus 4.6 ± 0.3 cm; p = 0.02) but not with recumbency (5.4 ± 0.2 versus 5.1 ± 0.3 cm; p = NS). There was a strong correlation between left ventricular end-diastolic dimension and stroke volume (r > 0.80) in all subjects. No significant changes in percent fractional shortening or left ventricular end-systolic dimension occurred in either position after cessation of training. Estimated left ventricular mass was 20% lower after 3 and 8 weeks of inactivity than when the subjects were conditioned (p < 0.05 for both). Thus, the endurance-trained state for upright exercise is associated with a greater stroke volume during upright exercise because of augmented preload. Despite many years of intense training, inactivity for only a few weeks results in loss of this adaptation in conjunction with regression of left ventricular hypertrophy.