Parvalbumin (PV) is found in the olfactory system, including the main olfactory bulb, and is thought to be one of the neuroactive substances in olfaction. Changes in PV immunoreactivity in the olfactory system during aging have not been examined. We investigated such changes in the main olfactory bulb (MOB) of the rat at postnatal month 1 (PM 1), PM 3, PM 6, PM 12 and PM 24. PV-IR neurons were almost completely restricted to the external plexiform layer. At PM 1 there were only a few PV-IR neurons; at PM 3, the number of PV-IR neurons was at its greatest but they were not well developed morphologically. At PM 6, the number of PV-IR neurons was similar to that at PM 3 and they had satellite somata with well-developed processes with many varicosities. By PM 12 the number of neurons and processes had declined, and by PM 24, they had fallen even further and the remaining processes had lost most of their varicosities. We conclude that age-related degeneration of PV-IR neurons in the MOB may reduce calcium buffering and affect olfactory function in senile species.