Polyamine levels were determined during pre- and postnatal development of the male and female mouse brain to assess their possible role during normal brain development. Tissue polyamine levels were increased during periods of neurogenesis and increased cell packing density (assessed by DNA levels) in all brain regions: cerebral cortex, cerebellum, hypothalamus, and rhombencephalon-midbrain. Tissue polyamine levels declined subsequently in a tissue-specific manner toward adult levels. In contrast, the polyamine concentrations per cell were decreased during periods of neurogenesis. A later increase in rhombencephalon-midbrain spermidine levels is consistent with the presumed association of this polyamine with myelin. No sex differences were found in these studies.