1. The total testicular content of RNA, DNA and protein was found to decrease sharply in hamsters with shortened photoperiod and in ground squirrels during the spring breeding season. 2. RNA and DNA per g testes were found to increase in both animals, while protein per g testes remained fairly stable. 3. Cell-free protein synthesis by testicular PMS during testicular regression remained constant when expressed per mg of testicular RNA, but decreased 75% when expressed per testes. 4. These findings suggest that decreases in testicular protein synthesis are due to a decrease in RNA content and not to alteration of the translational activity of the RNA.