In 11 healthy subjects (8 males and 3 females, age 21-59 yr) left ventricular end-diastolic (LVEDV) and end-systolic (LVESV) volumes were measured in the supine position by isotope cardiography at rest and during two submaximal one-legged exercise loads before and 1 h after acute plasma expansion (PE) by use of a 6% dextran solution (500-750 ml). After PE, blood volume increased from 5.22 +/- 0.92 to 5.71 +/- 1.02 (SD) liters (P < 0.01). At rest, cardiac output increased 30% (5.3 +/- 1.0 to 6.9 +/- 1.6 l/min; P < 0.01), stroke volume increased from 90 +/- 20 to 100 +/- 28 ml (P < 0.05), and LVEDV increased from 134 +/- 29 to 142 +/- 40 ml (NS). LVESV was unchanged (44 +/- 11 and 42 +/- 14 ml). Heart rate rose from 60 +/- 7 to 71 +/- 10 beats/min (P < 0.01). The cardiac preload [central venous pressure (CVP)] was insignificantly elevated (4.9 +/- 2.1 and 5.3 +/- 3.0 mmHg); systemic vascular resistance and arterial pressures were significantly reduced (mean pressure fell from 91 +/- 11 to 85 +/- 11 mmHg, P < 0.01). Left ventricular peak filling and peak ejection rates both increased (19 and 14%, respectively; P < 0.05). During exercise, cardiac output remained elevated after PE compared with the control situation, predominantly due to a 10- to 14-ml rise in stroke volume caused by an increased LVEDV, whereas LVESV was unchanged. CVP increased after PE by 2.1 and 3.0 mmHg, respectively (P < 0.05).2+ remained unchanged during exercise compared with rest after PE in