A combination of immunocytochemistry, in situ hybridization and ligand binding were used to investigate the localization of IGF-I and its receptor in the retina of diabetic and non-diabetic BB W-rats. Immunocytochemical localization revealed the presence of IGF-I in retinal pigment epithelium, ganglion cells, Muller cell processes and in microvessels. In most sites immunoreactivity was increased in the diabetic retina compared to that of non-diabetic BB W-rats. In microvessels, however, immunoreactivity was decreased in diabetes. In situ hybridization using an antisense IGF-I riboprobe provided evidence of IGF-I synthesis in all retinal layers with a similar grain density in diabetic and non-diabetic rats. Autoradiographic localization of IGF-I receptors, using [125 I]-IGF-I binding, demonstrated a diffuse localization in all retinal layers, with an increase in diabetic animals. These findings suggest that IGF-I synthesis is not altered in the diabetic retina, and that the increased immunoreactivity of IGF-I detectable in the various layers of the retina from diabetic rats may be due to an increased uptake of blood-derived IGF-I suggested by increased receptor density in diabetic rats.