Osteocytes from the proximal tibial metaphysis of mice were examined by electron microscopy at various time intervals after the incorporation of 1, 1.5, or 5 μCi/kg of body weight 224Ra. The animals were sacrificed 2 hr, 24 hr, or 5 days following administration of the short lived bone seeking radionuclide. Only the younger, osteoblast like osteocytes from the undecalcified trabecular bone were studied. Compared with the osteocytes from bone tissue of control animals, about half of the cells examined in specimens from animals treated with 224Ra showed ultrastructural alterations. In the nuclei of many osteocytes the chromatin was condensed. The mitochondria showed various signs of damage such as loss of cristae, swelling, and dissociation or disruption of both limiting membranes. Additionally, giant mitochondria were found. The rough surfaced endoplasmic reticulum was sometimes dilated and formed large cisternae. The Golgi complex was vacuolated. The pericellular spaces of lacunae containing severely damaged osteocytes were enlarged. It is thought that the early ultrastructural changes in osteocytes from the trabecular bone of mice resulted from the effects of direct irradiation upon the cells. Probably, all osteocytes which showed lesions were situated within the range of the α radiation emitted from the incorporated 224Ra which was randomly distributed in the mineralized bone matrix of the trabeculae. The mitochondria of the osteoblast like osteocytes appeared to be the cellular organelles most sensitive to the effects of 224Ra administration.