The effects of maternal exposure to a single dose of whole body irradiation (0.5 Gy) on gestational days (GD) 17, 18, 19, or 20 on the number of Purkinje and granule cells in the pyramis of the cerebellar cortex was examined in the offspring of Sprague Dawley rats at 21 and 28 days postnatally. The laterolateral distribution of both granule and Purkinje cells in the pyramis were significantly reduced (p < 0.001) from controls in rats irradiated on each of GD-17, 18, 19 and 20. There was a greater deficit in granule cell number with irradiation on GD-20 than on GD17 (p < 0.05). Purkinje cells were reduced in number with irradiation on GD-17 and GD-20; however, the decrease did not correspond to the degree of reduction in the number of granule cells. There was a greater reduction of both granule and Purkinje cells in the vermis with irradiation on GD-17, whereas on GD-20, both granule (p < 0.05) and Purkinje cells (p < 0.001) were more reduced in the lateral hemispheres. The GC/PC ratio was smaller in rats irradiated on GD-20 than on GD-17. The GC/PC ratio between the irradiated animals and the controls were relatively similar. The findings show that irradiation does not affect the population of granule cells directly, but rather indirectly. The reduction in the number of granule cells could be an indirect consequence of reduced critical interaction with Purkinje cells. These results suggest that a direct relationship exists between the proliferation, migration, development, and maturation of granule cells and their induction by Purkinje cells.