1.1. The incorporation of into protein of slices of testes of the adult rat, mouse, hamster, rabbit, guinea pig and dog has been studied following a 1 hr incubation period at temperatures of 26, 32, 37·5 and 44°C.2.2. In the rat, mouse and hamster, maximal incorporation of radioactive lysine into testicular protein occurred at 32°C.3.3. In the rabbit, guinea pig and dog, maximal incorporation of
... [Show full abstract] isotope into testicular protein occurred at 37·5°C.4.4. Measurements of the testicular temperatures of the rat, mouse and hamster were found to be lower than the corresponding testicular temperatures of the rabbit, guinea pig and dog.5.5. The data obtained indicate that optimal activity for testicular protein labeling from radioactive lysine in each species occurs at a temperature which is closest to that normally found in the scrotal sac of each animal.