Development of tumors in the liver of dogs exposed to gamma-quanta for 3 and 6 years at dose--rates of 0.0006, 0.0017 and 0.0034 Gy/day was studied 1-10 years after termination of the exposure. The results are summarized in this work. Tumors were found in 27 out of 89 dogs. The number of benign and malignant tumors was virtually the same (13 and 14). In the first place were hepatoma, among benign neoplasms (9 out of 13), and liver cancer, among malignant tumors (12 out of 14). Neoplasms were more frequent (32%) in exposed animals than in the controls (19%). Tumors were almost two times more frequent and developed earlier after 6 year- than after-3-year-irradiation.