Abstract Previous studies have reported the presence of binding sites for insulin-like growth factor I (IGF-I) in membranes prepared from isolated bovine adrenal medullary cells, and IGF-I was found to regulate the secretory function of bovine chromaffin cells. In the present study, binding sites for IGF-I have been localized in sections of bovine adrenal gland and on cultured bovine adrenal medullary cells, using [(125) l][Thr(59)]-IGF-l as the ligand in conjunction with qualitative autoradiographic techniques. Binding sites were present throughout the adrenal gland and were distributed evenly over all cortical cell layers and over both adrenaline and noradrenaline cell types in the medulla. They were also present at lower density over blood vessels and nerve bundles and over the capsule. The binding of radioligand was to a single class of sites with K(d) 0.61 nM, and was completely displaced by excess unlabelled [Thr(59)]-IGF-l and by insulin (Actrapid, K(d) 1.04muM). Binding sites were also identified on single cells in primary monolayer cultures of bovine adrenal medullary cells. More than 96% of the cells possessed binding sites, although only 85% of such cells were chromaffin cells, as previously determined from dopamine beta-hydroxylase immunohistochemical staining. The results suggest that IGF-I may affect the maturation, growth or function not only of adrenal chromaffin cells but also of many others cell types in this tissue.