To clarify the functional differences among G-proteins, we investigated the localization of Gi and Go in the olfactory bulb of rats by both immunohistochemical and immunochemical techniques, using purified antibodies specific to the alpha-subunits of Gi1 (Gi1 alpha), Gi2 (Gi2 alpha), and Go (Go alpha), respectively. We found that Gi2 alpha is localized exclusively in the accessory olfactory bulb, but it is present at only low levels in the main olfactory bulb. The unique pattern of immunoreactivity specific for Gi2 alpha and Go alpha within the glomeruli of the accessory olfactory bulb and the results of immunoassays indicate that the accessory olfactory bulb is divided into two parts: the anterior region is rich in Gi2, while the posterior region is rich in Go. These findings suggest that the accessory olfactory bulb has two different functions. In addition, we found that the concentration of Gi2 alpha in the accessory olfactory bulb increases during puberty and reaches the adult level at 12 weeks after birth, while that in the main olfactory bulb remains constant. By contrast, the concentrations of Go alpha in the accessory olfactory bulb and the main olfactory bulb increase with similar kinetics. These findings suggest that Gi2 is a key protein in signal transduction in the accessory olfactory bulb, and increases in its level seem to be related to sexual maturation.