Sprague-Dawley rats were exposed to beta rays of P32 for the study of late effects. Tumors of practically every type that can arise from skin and connective tissue appeared at 10 months to 1 year after exposure, irrespective of whether a single or repeated exposures took place. The results presented show a greater response in males than in females. The tumors showed in many instances a high degree of differentiation, with a tendency for the epithelium to form squamous cell and basal cell carcinomas, as well as adenomas, and for the connective tissue to form tendon, cartilage and even bone. Non-malignant abnormalities were also seen. These varied in type, consisting of accessory parts, such as extra claws on the toes, soft tissue extensions and chitinous projections from the feet and nose.