To study the role of insulin-like growth factor I (IGF-I) in mediating the growth-promoting function of GH, IGF-I expression was assayed in transgenic mice carrying either GH or GRF fusion genes. These mice exhibit enhanced growth as a result of high level ectopic expression of GH and have 2-fold elevation of both hepatic IGF-I mRNA levels and circulating IGF-I levels. Hepatic IGF-I mRNA levels are low at birth regardless of GH levels; they increase approximately 10-fold during postnatal development and become GH inducible 2 weeks after birth. The ontogeny of circulating IGF-I is similar. Accelerated growth in transgenic mice with GH fusion genes commences 3 weeks after birth despite high circulating GH levels at least as early as birth. In addition, endogenous mouse GH-secreting somatotroph cells in the pituitary are present in normal numbers at 3 weeks, but are undetectable 4 weeks after birth. Because the time at which IGF-I expression becomes GH responsive slightly precedes both the initiation of accelerated growth and the time when endogenous GH disappears, we conclude that IGF-I is directly involved in mediating the GH signal and that delayed induction of IGF-I gene expression is responsible, at least in part, for the delayed onset of other GH-responsive events.