The β-endorphin expression in the testis during the reproductive cycle was performed for the first time in the sand rat Psammomys obesus, a diurnal deserticole rodent belonging to the family of Gerbillidae and characterized by a seasonal reproductive function, with maximum activity in the winter and a rest from late spring through summer. For a clearer understanding of the seasonality control of the sexual cycle in this rodent, immunohistochemical and molecular study was performed in individuals at different stages of the cycle, in order to highlight the presence of the β-endorphin and how was regulated by testosterone. The results show that β-endorphin was only expressed in quiescent adults. The signal was expressed in germ cells, mainly in the spermatocytes I and at the level of the Leydig cells. The treatment with testosterone induces a partial decrease of the immunoreactivity both in germinal (50%) and interstitial (60%) compartments. The immunohistochimical results were confirmed by the western blotting technique,the β-endorphin, a band of 3.5kDa, was expressed only in quiescent adults. The band becomes less expressed after testosterone administration. The correlation between the testicular quiescence and the β-endorphin expression suggests that this peptide inhibits spermatogenesis. The testosterone does not seem to be the only regulatory factor of this testicular opioid peptide.