Abstract
Thymectomy was performed on the urodele amphibian Pleurodeles waltlii Michah. from the 5th to the 20th week of larvel development. Thymectomized animals were allografted at 7 months of age. The results obtained are as follows (1) Pleurodeles thymectomized before 10 weeks always tolerated allografts; (2) Pleurodeles thymectomized within 11 to 13 weeks generally rejected allografts in a chronic manner, but sometimes tolerated them; (3) normal rejections occurred in Pleurodeles thymectomized at 14 weeks, or later. Stage 52 of the development table (10 weeks of age) is a critical „end stage”︁ for the efficiency of thymectomy. The results are confirmed by the fate of second set allografts performed in all these animals.