Angiogenesis during lung development probably requires coordinated expression of mitogenic factors. While proliferation of proximal airway vessels occurs early in gestation, alveolar division is associated with important postnatal vessel growth. Using alternate splicing of two exons, smooth muscle cells in culture have been shown to express up to 3 isoforms of VEGF with varying endothelial mitogenic activities. We examined whether VEGF expression and alternate splicing is regulated during lung development. Using RNA blot hybridization and a murine VEGF cDNA, VEGF mRNA was detected in the fetal and postnatal rat lung. Highest levels were found in lungs from 8 day old rats with much lower levels detectable in fetal and adult lungs. To examine the pattern of alternate splicing of VEGF mRNA, we used RT-PCR to amplify VEGF cDNAs from lung mRNA of 8 day old rats. cDNAs corresponding to 121-, 165- and 189-aa VEGF isoforms were isolated. Using RNAse protection and isoform-specific cRNA probes, we found that mRNAs encoding 121- and 165-aa VEGF isoforms, which are secreted by smooth muscle cells and cause endothelial mitogenesis, were most abundant in the 8-day-old rat lung with lower levels detected in adult lung. Pulmonary levels of mRNA encoding 189-aa VEGF, which is not secreted or mitogenic, increased steadily during with age. These data suggest that alternate splicing of VEGF isoforms is regulated during development in the lung. Modulation of VEGF mRNA splicing may permit synthesis of mitogenic isoforms at a times of important postnatal vascular growth in the rat lung.