We examined whether an increase in stroke volume (SV) contributes to the increase in cardiac output that occurs when cardiac transplant patients (CT) exercise between 50% and 75% of maximum. Upright arm and leg exercise was performed by 13 CT and 10 normal controls. Cardiac output (CO2rebreathing), expired air, and catecholamine measures were obtained at rest and at 50% and 75% of maximum. From rest to 50% of maximum both normals and CT increased heart rate (HR) and SV during arm and leg exercise. In-normals, HR (arm = +17 ± 2 -min−1; leg = +21 ± 3 min−1) was increased further at 75% of maximum but not SV (arm = +3 ± 4 ml; leg = +2 ± 8 ml). In CT, further increases in both HR (arm = +9 ± 1 ml; leg = +13 ± 1 min−1) and SV (arm = +12 ± 5 ml; leg = +12 ± 3 ml) were observed at 75% of maximum. During leg exercise plasma norepinephrine was increased more in CT than in normals. Between 50% and 75% of maximum, an increase in SV is a more important mechanism for increasing cardiac output during upright arm and leg exercise in CT than in normals.