Patients who have undergone Fontan's operation are known to have impaired cardiac output response to dynamic exercise. This may be due to either poor cardiac function or a limited ability to mobilize blood from capacitance vessels due to increased resting venous tone. We tested the latter hypothesis by determining venous vascular capacitance at rest and during orthostatic stress produced by lower body negative pressure (LBNP) in 6 subjects who had undergone the Fontan operation and 6 healthy age-, sex-, height-, and weight-matched controls. Resting blood volume was similar for Fontan and control subjects (79 ± 6 vs 70 ± 3 ml/kg body weight, respectively), while central venous pressure (CVP) was elevated in Fontan subjects (18.4 ± 1.0 vs 3.5 ± 0.9 mm Hg, p < 0.05). Forearm venous capacitance at a distending pressure of 40 mm Hg was less in Fontan subjects than in controls (2.6 ±0.1 vs 3.9 ± 0.5 ml/100 ml), while resting plasma norepinephrine level was elevated in Fontan subjects (255 ± 28 vs 144 ± 9 pg/ml, p < 0.05). The increase in calf volume (1.6 ± 0.2 vs 2.3 ± 0.2 ml) and decrease in CVP (−5.0 ± 0.5 vs −6.7 ± 1.1 mm Hg) during −30 mm Hg LBNP were smaller for Fontan than control subjects (p < 0.05). Reduced forearm venous capacitance and diminished pooling of blood into capacitance vessels of the leg during orthostaric stress indicated higher venous tone in Fontan than control subjects. We propose that increased resting venous tone in Fontan subjects may limit their ability to mobilize blood from capacitance vessels during exercise and may contribute to impaired cardiac output response.