Fertilized Japanese quail eggs were continuously exposed to 2.45 GHz microwave radiation from day 1 through day 12 of incubation at a power density of 5.0 +/- 0.52 mW/cm2 (mean specific absorption rate = 4.03 mW/gm). In the experiment with embryos, irradiated and control embryos were removed from eggs on day 12, 13 or 14 of incubation and the cerebella were histologically examined. In order to examine the long-term effect of microwave radiation during embryogenesis on the cerebellum, some of the quail were allowed to hatch and were reared to eight weeks of age. Their cerebella were histologically examined, and the extent of dendritic arbores, the length of the stem of the primary dendrite, and the size of the perikaryon of Purkinje cells were measured in Golgi-Cox impregnated sections. In the irradiated embryos, a slight developmental retardation was found in the cerebellar cortices in terms of several morphological parameters. The effects included the growth and subsequent decline of the external granular layer, the growth of the molecular layer, the cellular differentiation and the alignment of Purkinje cells, and the accumulation of granule cells beneath the Purkinje cell layer, as well as lower body and brain weights. In the eight-week-old quail, no significant differences were noted between irradiated and control cerebella in the morphological measurements of Purkinje cells.