To clarify the physiology of venous return (Q(vr)) in Fontan circulations, venous return conductance (G(vr)) and mean circulatory filling pressure (P(mcf)) were determined in pentobarbital sodium-anesthetized pigs. Relationships between Q(vr) and right (biventricular, n = 8) or left (Fontan, n = 8) filling pressures are described by straight lines with significant correlation coefficients. Estimated P(mcf) values were correlated with observed P(mcf) values in either circulations (P </= 0.02). G(vr) was smaller in Fontan than in biventricular circulations (4.51 +/- 0.36 vs. 7.83 +/- 0.69 ml. min(-1). kg(-1). mmHg(-1), P = 0.002) and inversely correlated with pulmonary vascular resistances in Fontan circulations (P = 0.01). Estimated P(mcf) (20.5 +/- 1.4 vs. 11.1 +/- 0.9 mmHg, P = 0.001) and observed P(mcf) (21.8 +/- 1.3 vs. 10.6 +/- 0.8 mmHg, P < 0.001) were higher in Fontan versus biventricular circulations, respectively. Pulmonary artery pressure in Fontan circulations was correlated with either P(mcf) (P < or = 0.04). We conclude that in Fontan circulations 1) pulmonary vascular resistances induce a proportional decrease in G(vr); and 2) volume loading, while increasing P(mcf) (similar to pulmonary artery pressure), allows the gradient for Q(vr) to increase and maintains systemic blood flow at a biventricular level.