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Previous studies suggest that pulmonary capillary distensibility and recruitment may differ in lambs and sheep. To study the effect of pulmonary blood flow (PBF) and vascular pressure on capillary hemodynamics in lambs and sheep we measured the diffusing capacity for carbon monoxide (DLCO) as an index of pulmonary capillary blood volume during a baseline period, after increasing PBF, and during left atrial hypertension. In the lamb, DLCO did not change significantly either with a 65% increase in PBF or with an increase in left atrial pressure (Pla) of 1.33 kPa at constant PBF. In the sheep on the other hand, doubling PBF led to a 28% increase in DLCO (P less than 0.02), and an increase in Pla of 1.87 kPa at constant PBF led to a 19% increase in DLCO (P less than 0.01). These results suggest that the neonatal lamb has a nearly fully recruited and relatively non-compliant pulmonary capillary bed at rest, unlike the adult sheep which can respond to hemodynamic changes with distension and recruitment of the pulmonary capillary bed.