Work from our laboratory has shown that adult male rats have 19% more neurons than female rats in the binocular region and 18% more in the monocular region of the primary visual cortex (Reid and Juraska [1992] J Comp Neurol 321:448-455; Nuñez et al., [1999] Soc Neurosci Abstr 25:229). In the current experiment, we investigated whether cell death in male and female rats (postnatal days 2-35) contributes to the formation of these differences. Using stereological techniques, we investigated neuron density along with pyknotic and apoptotic (TdT-mediated deoxyuridine triphosphate nick end-labeled) cell density in the developing posterior cortex (future primary visual cortex). Although no sex differences in neuronal density were found in early development, we observed a differential time course of cell death between the sexes. Consistent with earlier reports, males displayed a rapid rise in cell death, with a peak on day 7 followed by a sharp decline to negligible levels by day 15. Females, however, displayed moderate peaks of cell death on days 7 and 11, with the persistence of low-to-modest levels until day 25. Similar patterns were obtained from both pyknotic and apoptotic cell quantification. Also, a formula was developed to estimate the percentage of cells that die during development and the amount of time a dying cell is visible. This study demonstrates that there is a prolonged period of cell death in the posterior cortex of developing female rats that appears to result in more cell death in females than males. This may be an important mechanism by which the sex difference in adult neuron number is created.