Z-DNA has been considered a labile but essential structural form of DNA in recombination and gene expression, two significant activities in mammalian seminiferous epithelium. The present study has utilized the recrudescing testes of Mesocricetus brandti to study in detail the potential Z-DNA sites in specific testicular cell types as detected by an immunoprobe. Testicular regression was physiologically induced by modifying environmental photoperiods and/or temperature. Partial atrophy of seminiferous epithelium occurred in all experimental groups but Sertoli cells persisted throughout regression. Recrudescence of testicular activity was marked in all experimental groups by characteristic sequences of reappearance of potential Z-DNA sites to a final positive or negative mature state of the cell type. It is suggested that Z-DNA is a functionally important from of DNA in many cell types of the active seminiferous epithelium of the Turkish hamster, and perhaps other mammals.