The effect based upon changes of body weight and effect of X-ray irradiation on tryptophan pyrrolase (TPO) activity in liver homogenate of albino rats was studied. 1. The average TPO activity of control group of 33 rats weighing 95 ~ 172g was 2.372±0.165μ moles kynurenine/g protein/hr. 2. Correlation between body weight of 33 rats and its TPO activity was not showed practically(r=0.011). 3. TPO activity of whole-body X-ray irradiated rats (700r) was increased about double at 3hours and increased for a period of 4 days after irradiation, but after 6 days it was decreased gradually. This result is that whole-body X-ray irradiation showed dual effect on TPO activity in rat liver.