Endothelial cells perform a number of important functions including release of vasodilators, control of the coagulation cascade, and restriction of solutes and fluid from the extravascular space. Regulation of fluid balance is of particular importance in the microcirculation of the lung where the loss of endothelial barrier function can lead to alveolar flooding and life-threatening hypoxemia. Significant heterogeneity exists between endothelial cells lining the microcirculation and cells from larger pulmonary arteries, however, and these differences may be relevant in restoring barrier function following vascular injury. Using well-defined populations of rat endothelial cells harvested from the pulmonary microcirculation [pulmonary microvascular endothelial cells (PMVEC)] and from larger pulmonary arteries [pulmonary artery endothelial cells (PAEC)], we compared their growth characteristics in low serum conditions. Withdrawal of serum inhibited proliferation and induced G0/G1 arrest in PAEC, whereas PMVEC failed to undergo G0/G1 arrest and continued to proliferate. Consistent with this observation, PMVEC had an increased cdk4 and cdk2 kinase activity with hyperphosphorylated (inactive) retinoblastoma (Rb) relative to PAEC as well as a threefold increase in cyclin D1 protein levels; overexpression of the cdk inhibitors p21Cip1/Waf1 and p27Kip1 induced G0/G1 arrest. While serum withdrawal failed to induce G0/G1 arrest in nonconfluent PMVEC, confluence was associated with hypophosphorylated Rb and growth arrest; loss of confluence led to resumption of growth. These data suggest that nonconfluent PMVEC continue to proliferate independently of growth factors. This proliferative characteristic may be important in restoring confluence (and barrier function) in the pulmonary microcirculation following endothelial injury.