We measured steady-state lung lymph flow, lymph protein flow, and simultaneous pulmonary vascular pressures in 12 1-wk-old unanesthetized lambs and compared these measurements to those of previous studies, performed under similar conditions, on nine awake adult sheep. The purpose of these experiments was to compare newborn and adult sheep with respect to transvascular filtration of fluid and microvascular permeability to plasma proteins. We prepared the lambs surgically to isolate and collect lung lymph and measure average pulmonary arterial and left atrial pressures, allowing at least 2 days for the lambs to recover from surgery before studies began. Lambs had higher pulmonary arterial and left atrial pressures, lower lymph and plasma protein concentrations, and 57% more lymph flow per gram of dry bloodless lung than sheep; the difference in protein flow between lambs and sheep was not significant. Protein concentration in lymph relative to that in plasma was significantly lower in lambs than in sheep; but the ratio of albumin concentration to globulin concentration in both lymph and plasma was almost identical in the two groups of animals. Extravascular lung water per gram of dry bloodless lung was greater in lambs (4.82+/-0.11 g) than in sheep (4.45+/-0.08 g), but there was no histologic evidence of pulmonary edema in either group of animals. These findings suggest that lambs have more transvascular filtration of fluid per unit lung mass than sheep, but that microvascular sites for protein exchange do not differ appreciably in lambs and sheep. To test this conclusion, we measured steady-state lymph flow in three lambs before and after raising pulmonary microvascular pressure by rapid intravenous infusion of saline. Lymph flow increased as a function of the net transvascular driving pressure (hydraulic pressure gradient-protein osmotic pressure gradient). This response was almost identical to that of four sheep with pulmonary microvascular pressure augmented by inflation of a balloon in the left atrium. In eight lambs we measured the time for intravenously injected (125)I-albumin to equilibrate in lymph at half the specific activity of plasma: the protein tag equilibrated faster than in sheep. This difference could be explained partly by the higher pulmonary arterial and left atrial pressures of lambs than sheep, and possibly by the presence of more microvascular sites for protein exchange relative to the volume of distribution of protein in the lung of the younger animals.