Exposure of fetal rats to X-irradiation on gestational day 15 resulted postnatally in dose-related effects on body weight, growth of forebrain structures, and branching of dendrites of caudate neurons. Rats were followed for 4 months postnatally after 125, 75, 50, or 25 R whole-body irradiation to the dam. Significant decreases in body weight were present at birth after the three high doses and continued as long as 4 months after 125 or 75 R. Decreased thickness of the cerebral cortex and decreased area of the caudate nucleus were also seen. Cortical thickness was reduced by 125 R to half the size of the control cortex and the caudate nucleus to two-thirds of the control. Significant decreases were present to 50 R. Dendritic branching was reduced in caudate neurons by 125 R but not in the basilar dendrites of cortical pyramidal cells. No reduction in number of cortical synapses was seen from electron micrographs of cortical layers 1 or 5. The effect on the cerebral cortex was interpreted as a loss of neurons with retention of branching and synaptogenesis of remaining neurons. In contrast, the caudate nucleus, which develops somewhat before the cerebral cortex, showed effects as a consequence either of direct damage to caudate neurons or of reduced neuropil from reduced afferent input.