Rhesus monkeys were irradiated by ,60,Co γ ray exposure under different conditions, and comparative studies on their chromosome aberrations have been made. The dose rate of single acute irradiation was 223 mGy/min, the dose rate of fractionated cumulative irradiation was 223 mGy/min, and the dose of 0.25 Gy was delivered once a week, the dose rate of chronic continuous irradiation was 0.26 mGy/min for 190 min, 5 days per week. The total cumulative dose for each group was 2.0 Gy.,It was found that the types of chromosome aberrations were mainly dicentric aberrations and acentric fragments in single acute irradiation, and acentric fragments in both fractionated cumulative and chronic continuous irradiation groups. The frequencies of chromosome aberrationns in these groups increase linearly with increasing dose. The degree of chromosome damage in single acute irradiation group was much greater than those in the other groups, but degree and characteristics of chromosome damage between fractionated cumulative and chronic comtinuous irradiation groups was not significantly different. Acentric fragments in lymphocyte chromosomes ceased to increase when the cumulated dose in fractionated cumulative or chronic continuous irradiation reached to more than 1.0 Gy. The frequencies of chromosome aberrations basically decreased to control level within a year following the termination of irradiation.