There is disagreement in the literature about the time required for hypoxic constriction of pulmonary vessels to reach its full intensity. Some studies suggest that only minutes are required, others that several hours are needed. We examined the time course over 6 h of changes in pulmonary shunt (as a fraction of cardiac output) following induction of unilateral hypoxia by collapse or liquid filling of the left lung in 47 anesthetized rabbits. The time course was examined at four degrees of lung inflation: during collapse and at airway pressures of 0.3 kPa, 0.6 kPa, and 0.9 kPa. The respective volumes (mean +/- SD) of the liquid-filled lung were estimated to be 6.4 +/- 1.0, 12.8 +/- 2.5, and 15.8 +/- 1.6 ml/kg body weight (BW). During sustained hypoxia (the period from 150 to 360 min after inducing hypoxia), shunt declined at a slow linear rate of 2.37 x 10(-4)/min, which was independent of lung inflation (p = 0.65 analysis of variance [ANOVA]) and significantly different from zero (p < 0.001). The stability of cardiac output in this animal model, as measured sequentially by thermodilution, was confirmed in a further 20 animals. The experiments provide evidence for a slow intensification of blood-flow diversion at a rate that does not depend upon the degree of lung inflation. Whether this change is a feature of hypoxic constriction itself, or some modulation of it, remains unclear.