The earliest and most striking proliferative changes in the neonatal pulmonary arterial wall occur in the adventitia where the fibroblast resides. The protein kinase C (PKC) pathway is developmentally regulated and important in vascular cell growth. We tested the hypothesis that developmental differences in growth of pulmonary artery adventitial fibroblasts would be detectable in vitro and dependent on PKC. Fibroblasts were isolated from bovine fetal, neonatal, and adult pulmonary arteries. Growth was measured by [3H]thymidine incorporation and cell counts. Under serum-stimulated conditions, fetal and neonatal pulmonary artery fibroblasts grew faster and reached higher plateau densities than adult cells. Increased growth of fetal cells in vitro was dependent on time of harvest during fetal life (early > late). Under quiescent conditions, fetal and neonatal fibroblasts had increased DNA synthesis compared with adult cells in response to the PKC agonist phorbol 12-myristate 13-acetate. To test whether the developmental differences in fibroblast growth were dependent on PKC, three different inhibitor strategies were used (dihydrosphingosine, phorbol-ester-induced downregulation, and heparin). Fetal and neonatal fibroblasts were more susceptible than adult cells to each antagonist strategy. Finally, we measured whole cellular PKC catalytic activity and found it correlated with growth and susceptibility to PKC inhibition (i.e., fetal PKC activity > neonatal > adult). We conclude that PKC-dependent developmental differences in growth of pulmonary artery fibroblasts are detectable in vitro and that the enhanced growth capacity of fetal and neonatal cells may contribute to the dramatic adventitial thickening seen in vivo after hypoxic exposure in the neonatal calf.