The purpose of this article was investigation the development of the posterity rat (line Wistar) in the postnatal ontogenesis after combined and separate action of gamma-radiation (1.25 Gy at 20 days) and hyperthyroidism, provoked by toxic thyroid doses of 100 mg thyroidin per rat. It was shown that hyperthyroidism did not affect physical development of the irradiated posterity, but in a long-time period (7 month) it was revealed a decrease in ability to the training of posterity rats after the combined action of the factors.