In order to estimate end effects of chronic prolonged gammairradiation of dogs, an exposure of 80 animals to irradiation was terminated and they were followed up closely. Out of 80 animals 30 dogs (1st series) were irradiated for 3 years and 50 dogs (II series) for 6 years. The dogs were exposed to irradiation at doses of 21 to 190 rad per year. Out of the total number of animals 22 dogs died. Post-mortem examinations showed neoformations in 13 animals (7 malignant and 12 benign neoformations). The highest number of tumors developed in dogs of the II series (10 out of 11) one-two years after irradiation (6 malignant tumors--malignant pheochromocytoma of adrenals; malignant adenoma of the hypophysis: polymorphocellular sarcoma of the liver; leucomyosarcoma of the uterus; bladder cancer; breast cancer; and 10 benign tumors--pancreatic adenoma; liver angioma; 2 papillary adenomas of the prostate; 3 renal adenomas; lipoma; polyps of the gall-bladder). Animals of the 1st series displayed 3 neoformations (1 malignant tumor--bladder tumor and 2 benign tumorsliver hepatoma and spleen angioma) 4--5 years after irradiation.