Differentiation and maturation of the photoreceptor outer segments are key steps in the development of the visual system. Morphological studies presented here show that the cow and human are nearly identical in the timing of outer segment appearance during fetal development, implying that the bovine retina is a good model system for the final stages of human photoreceptor development. To study photoreceptor maturation, rhodopsin and interphotoreceptor retinoid-binding protein (IRBP) were quantified by ELISA in a developmentally staged series of fetal bovine retinas. In addition, their localization within these retinas was determined by immunogold electron microscopy. Rhodopsin, as detected by antibodies directed against either the N- or C-terminal portions of the molecule, is first found at about 5.5 months gestation. It is first detected on the plasma membrane of the immature cilia and on the earliest emergent outer segment membrane, even before organized disk membranes are apparent. In contrast, whereas rhodopsin levels and outer segments are nearly undetectable before 5 months gestation, IRBP accumulates to a significant level (4-5% of the adult) as early as 3 months gestation. Immunogold electron microscopy confirmed this finding, with localization of IRBP predominantly in the subretinal space.