Alveolar type-II cells are responsible for alveolar epithelial cell proliferation during growth and development and in response to lung injury. Based on the observation of abnormal lung development in rachitic rat pups and the expression of receptors for vitamin D by fetal alveolar epithelial cells, the present study examined the influence of 1,25-dihydroxy vitamin D (DHD) on the proliferation of primary cultures of fetal, neonatal and adult alveolar epithelial cells. The ontogony of vitamin D responsiveness was examined, using fetal (days 18, 19 and 22 = term), neonatal (days 7 and 18) alveolar epithelial cells as well as adult alveolar type-II cells. Maximal stimulation of [3H]thymidine incorporation occurred in neonatal d18 cells: (250 +/- 4.8%, n = 4, P < 0.05). Incubation of adult type-II cells, in the presence of 10(-9) M DHD increased thymidine incorporation into DNA (149.1 +/- 33.2%, mean +/- S.E., n = 3, P < 0.001) compared to control cells maintained in basal medium. Exposure to DHD also increased thymidine incorporation after stimulation with a mixture of conventional progression factors (insulin (10 micrograms/ml) (I), cholera toxin (10 micrograms/ml) (C) and EGF (20 ng/ml) (E)) (349.4 +/- 42.9% vs. 213.5 +/- 23.6%, n = 6, P < 0.005). Autoradiographic labeling indices of adult type-II cells increased from 3.1 +/- 0.6% for cells cultured in basal medium to 7.2 +/- 1.7% in cells exposed to DHD from the time of plating and I, C, E from 20-68 h in culture (n = 4, P < 0.05). Although no increase in the number of adult type-II cells was observed in these experiments, flow cytometric analysis of nuclear DNA content revealed an increased proportion of cells in the S and G2 phases of the cell cycle (basal: S = 2.6%, G2/M = 3.0%, DHD+GF: S = 4.7%, G2/M = 5.6%, P < 0.05 for each comparison). These data demonstrate that vitamin D3 is a growth factor for alveolar type-II cells and suggest the possibility that local elaboration of vitamin D may provide a novel mechanism of modulation of epithelial proliferation in the context of lung development and repair.