The distribution and the levels of Gi1 (plus Gi3), Gi2, and G(o) in rat retina were studied immunohistochemically and immunochemically during development. At embryonic day (E) 15, Gi1 alpha/Gi3 alpha was observed in the inner layer of the neural retina, the future nerve fiber layer (NFL), while Gi2 alpha was observed both in the inner and outer layers of the neural retina. No immunoreactivity for G(o) alpha was observed. At E18, Gi1 alpha/Gi3 alpha and Gi2 alpha appeared in the inner plexiform layer (IPL), while G(o) alpha was faintly immunoreactive only in the NFL. At birth, Gi2 alpha/Gi3 alpha and G(o) alpha appeared in the ganglion cell layer. Gi2 alpha was intensely immunoreactive in the NFL and IPL. At postnatal day (P) 10, the inner portions of the retina, from the NFL to the outer plexiform layer, were immunoreactive to Gi1 alpha/Gi3 alpha, Gi2 alpha, and G(o) alpha. Gi1 alpha/Gi3 alpha and G(o) alpha were distributed characteristically in a laminated pattern in the IPL, but Gi2 alpha was present homogeneously in the IPL. At P12, Gi2 alpha appeared in the outer nuclear layer. As the postnatal days advanced, the laminated pattern of immunoreactivity to G(o) alpha in the IPL became diffuse, but immunoreactivity to Gi1 alpha/Gi3 alpha remained. The results of enzyme immunoassays showed that the concentration of G(o) alpha increased rapidly from P10 to P15 and reached almost the adult level at P20-P30, while Gi2 alpha decreased until P15 and was almost constant thereafter. These results showed that the distribution of Gi1 alpha/Gi3 alpha, Gi2 alpha, and G(o) alpha differs during development, suggesting that each G protein in the developing retina has a unique function.