An in vivo gene transfer technique for living mouse testes was used to develop a novel transient expression assay system for transcriptional regulatory elements of spermatogenic specific genes. The combination of DNA injection into seminiferous tubules and subsequent in vivo electroporation resulted in an efficient and convenient assay system for gene expression during spermatogenesis. The transfer of the firefly luciferase reporting gene driven by the Protamine-1 (Prm-1) enhancer region revealed a significant increase in the activity of the reporter enzyme. Histochemical studies of the transfer of the lacZ gene driven by the Prm-1 enhancer showed specific lacZ expression only in haploid spermatid cells in adult testes, corresponding with the expression pattern of endogenous Prm-1. We were able to detect long-lasting transgene expression in the transfected spermatogenic cells. A group of spermatogenic differentiating cells maintained the transfected lacZ expression after more than 2 mo of transfection, suggesting that spermatogenic stem cells and/or spermatogonia could also incorporate foreign DNA and that the transgene could be transmitted to the progenitor cells derived from a transfected proliferating germ cell.