The function of insulin-like growth factors I and II (IGF-I and IGF-II) in embryogenesis is unknown. To investigate the ontogeny of IGF gene expression during mammalian development we used a highly sensitive and specific solution-hybridization assay to determine the steady state levels of IGF mRNAs during midgestation in the rat. IGF-I mRNA can be detected as early as day 11 of embryonic development and rises 8.6-fold over the ensuing 48 h. By contrast IGF-II mRNA is relatively constant over days 11-14 of gestation. These observations suggest that both IGFs may play important roles in early fetal development.