Enhanced left-ventricular (LV) compliance is a common adaptation to endurance training. This adaptation may have differential effects under conditions of altered venous return. The purpose of this investigation was to assess the effect of cardiac (un)loading on right ventricular (RV) cavity dimensions and LV volumes in endurance-trained athletes and normally active males. Eight endurance-trained (Vo_2max, 65.4 ± 5.7 ml·kg⁻¹·min⁻¹) and eight normally active (Vo_2max, 45.1 ± 6.0 ml·kg⁻¹·min⁻¹) males underwent assessments of the following: 1) Vo_2max, 2) orthostatic tolerance, and 3) cardiac responses to lower-body positive (0–60 mmHg) and negative (0 to −80 mmHg) pressures with echocardiography. In response to negative pressures, echocardiographic analysis revealed a similar decrease in RV end-diastolic cavity area in both groups (e.g., at −80 mmHg: normals, 21.4%; athletes, 20.8%) but a greater decrease in LV end-diastolic volume in endurance-trained athletes (e.g., at −80 mmHg: normals, 32.3%; athletes, 44.4%; P < 0.05). Endurance-trained athletes also had significantly greater decreases in LV stroke volume during lower-body negative pressure. During positive pressures, endurance-trained athletes showed larger increases in LV end-diastolic volume (e.g., at +60 mmHg; normals, 14.1%; athletes, 26.8%) and LV stroke volume, despite similar responses in RV end-diastolic cavity area (e.g., at +60 mmHg: normals, 18.2%; athletes, 24.2%; P < 0.05). This investigation revealed that in response to cardiac (un)loading similar changes in RV cavity area occur in endurance-trained and normally active individuals despite a differential response in the left ventricle. These differences may be the result of alterations in RV influence on the left ventricle and/or intrinsic ventricular compliance.