In response to extracellular stimuli, the phenotype of a cell changes rapidly. In these processes genes whose induction occurs in the preence of protein synthesis inhibitors are referred to as immediate early response genes(IEG). IEGs encode growth factors, growth factor receptors, cytoskeletal structures, and transcription factors. Immediate-early transcription factors comprise several “families”: fos, jun and Egr. While Egr has been strongly implicated in the development of the immune response, its role in other developing systems remains largely unknown. Mammalian lung morphogenesis, in preparation for air breathing after birth involves increased proliferation of cells and rapid induction of many diverse genes. It was therefore of interest to investigate the developmental expression of Egr-1, c-fos and c-myc genes in the developing fetal rat lung. We observed an abundance of Egr-1 message(1.8 Kb) in the fetal rat lungs at 19 and 20 days (d) of gestation, after which, there is a drop in transcription at 21d. c-myc is transcribed maximally at 19-21 d gestation followed by a drop at 21d. c-fos, which has a broader range of action, is transcribed 19-21 d and decreases at term. To identify the specific lung cells involved in Egr-1 gene expression, we isolated RNA from fetal lung fibroblasts and epithelial cells from 18-21d fetal rat lungs and using Northern hybridization, assessed the abundance of Egr-1 message in these cells. Although both the cell types expretal lung fibroblasts demonstrated a developmental expression of the gene, with a peak expression occurring at 19-20 d of gestation followed by a decline in expression thereafter. Fetal lung epithelial cells, on the other hand, expressed the gene constitutively throughout gestation. These data suggest a possible role of these immediate early genes in the regulation of lung proliferation during gestation.