The granule cell population of the dentate gyrus is produced predominantly during the postnatal period in rats. Previous studies have shown that experimental increases in the levels of adrenal steroids suppress the proliferation of granule cell precursors during the first postnatal week, the time of maximal neurogenesis in the dentate gyrus. These findings raise the possibility that stressful experiences that elevate adrenal steroid levels may inhibit the production of granule neurons, and thus alter the development of the dentate gyrus. To test this possibility, we exposed naive rat pups to the odors of a known predator, adult male rats, and examined both plasma corticosterone levels and the number of 3H-thymidine labeled cells in the dentate gyrus. A single exposure of rat pups to adult male rat odor elevated corticosterone levels immediately and diminished the number of 3H-thymidine labeled cells in the granule cell layer by 24 h later. These results suggest that stressful experiences suppress the production of granule neurons in the developing dentate gyrus.