Neurogenesis occurs in the dentate gyrus of the hippocampus throughout the life of a rodent(1-4), but the function of these new neurons and the mechanisms that regulate their birth are unknown, Here we show that significantly more new neurons exist in the dentate gyrus of mice exposed to an enriched environment compared with littermates housed in standard cages. We also show, using unbiased stereology, that the enriched mice have a larger hippocampal granule cell layer and 15 per cent more granule cell neurons in the dentate gyrus.