Ethanol exposure during development is particularly deleterious to cerebellar Purkinje cells and granule cells, but the mechanism(s) underlying this sensitivity and the variables which affect it remain unknown. One important variable that has not been fully investigated, is the timing of the ethanol exposure. Ethanol exposure during the brain growth spurt causes a differential loss of Purkinje cells across the 10 lobules of the vermal cerebellum. However, whether or not changing the timing of the ethanol exposure during the brain growth spurt alters the extent and location of the loss of Purkinje cells within the cerebellar vermis has not been investigated. Moreover, the loss of cerebellar granule cells has been shown to parallel the loss of Purkinje cells, leading to the conclusion that the loss of granule cells occurred as a function of the loss of their targets, the Purkinje cells. The purpose of this study was to address both issues. Male rat pups were exposed to ethanol, via an artificial-rearing method, during one of the following 2-day time periods: postnatal days (PD) 4-5, 5-6, 6-7, 7-8, 8-9, 9-10, or 12-13. Gastrostomy control (GC) and suckle control (SC) groups also were included. All pups were sacrificed on PD21. The number of Purkinje cell nuclear profiles from three vermal sections were counted in all groups, while the number of granule cell nuclear profiles in the ten lobules was estimated from pups in selected groups. No loss of Purkinje cells was observed in pups exposed to ethanol on PD7-8 or at any of the later exposure times. Additionally, among the three exposure groups in which significant Purkinje cell loss was observed (PD4-5, PD5-6 and PD6-7), seven lobules exhibited significant differences particularly between the PD4-5 and PD6-7 groups. The group with the greatest loss of Purkinje cells (PD4-5) also was the group with the greatest loss of granule cells. A significant loss of granule cells did not occur without a corresponding loss of Purkinje cells. The loss of both the Purkinje and granule cells was affected by the timing of the ethanol exposure, and that the extent and the location of Purkinje cell loss were extremely sensitive to the effects of the timing of the ethanol exposure.