To localize mRNAs for insulin-like growth factor (IGF)-I, IGF-I receptor (IGF-IR), and IGF binding protein (BP)-1 to IGFBP-6 in the rat eye. cDNA sequences for IGF-I, IGF-IR, and IGFBP-1 to IGFBP-6 were used to synthesize 35S-CTP labeled antisense and sense probes for in situ hybridization on 5-microns sections of the rat eye, including the retina, choroid, sclera, ciliary body, and cornea. IGF-I mRNA was demonstrated over ganglion cells of the retina and endothelial cells of the choroid and ciliary processes. IGF-IR mRNA showed more extensive distribution, localizing to the retinal ganglion cell layer, inner nuclear layer, and outer limiting membrane and also the outer nonpigmented epithelium of the ciliary processes and cornea, conjunctiva, and lens. IGFBP-2 mRNA localized to outer nonpigmented epithelia of the ciliary processes and the germinal layer of corneal epithelium as well as iris, conjunctiva, and sclera. Messenger RNAs for IGFBP-3 to IGFBP-6 localized to choroidal endothelial cells and chromatophores and also to the inner pigmented epithelium of the ciliary processes. Messenger RNAs for IGFBP-5 and IGFBP-6 were seen in the inner and outer nuclear layers of the neural retina. IGFBP-1 mRNA was not detected within the rat eye. Using in situ hybridization, we have demonstrated mRNAs for IGF-I, IGF-IR, and IGFBP-2 to IGFBP-6 in specific histologic layers of the retina, choroid, ciliary body, and cornea in the rat. The characterization of the IGF system in vivo suggests specific roles in the normal eye and provides a basis for studying the IGF system in eye pathology.