Insulin-like growth factor I (IGF-I) is a mitogenic polypeptide that is thought to play, under the control of growth hormone, a role in fetal development as well as post-natally. The direct effect of growth hormone on the regulation of the expression of IGF-I gene was examined in adipose Ob1771 cells. Growth hormone regulates the abundance of multiple species of IGF-I mRNAs of 15, 7.5, 1.5 and 0.8 kb in a differentiation-dependent manner. The regulation of IGF-I gene expression is strikingly rapid (less than 2 h), reversible and takes place primarily at transcriptional level. Thus growth hormone can increase the cellular content of IGF-I mRNA encoding for a protein which could be involved in a paracrine/autocrine action during adipose tissue development.