Several researchers have determined that pulmonary capillaries in young mammals are significantly more permeable to water and lipid-insoluble solutes than are the pulmonary capillaries in the adult counterparts. Our purpose was to quantify the microvascular fluid flux parameters in 2- and 4-week-old rabbit lungs and to compare our results with adult and newborn data previously reported. Using an isolated, Ringer-perfused lung preparation, we measured the values for the microvascular filtration coefficient and the reflection coefficients for NaCl and albumin. We found a filtration coefficient value of 2.57 × 10-4 ml/sec-cm H2O-m2 for the 2-week-old population. For the 4-week-old rabbit we evaluated the filtration coefficient to be 3.18 × 10-4 ml/sec-cm H2O-m2. These values are normalized for the estimated exchange area, and neither is significantly different from the adult value. The reflection coefficients for NaCl and albumin were measured using an osmotic transient technique. Correcting the albumin data for flow across the endothelial cell membrane, we arrived at the following values for the pulmonary microvascular reflection coefficient of 2- and 4-week-old rabbits, respectively: σcapalb = 0.17, and σcapalb = 0.23. Based on these values we estimated the equivalent pore radii of large pores in the pulmonary microvascular wall to be approximately 175 Å in the 2-week-old, and 145 Å in the 4-week-old rabbits. These results indicate that in the rabbit Kf/m2 has reached the adult value by 2 weeks of age; however, the number of pores increases, and the pore radius decreases, continuously even beyond 4 weeks of age.