en Original Article Original Article <strong>Background:</strong> Diazinon (DZN) is an organophosphate insecticide which is used worldwide in agriculture. The exposure to this chemical might lead to damages to the living systems.<br> <strong>Objective:</strong> The present study was done to investigate the effects of diazinon on the structure of testis and levels of sex hormones in adult male mice. <strong>Materials and Methods:</strong> For this experiment, the mature male mice divided into three groups; Control (no injection), sham (corn oil injection) and DZN (diazinon was administrated at dose of 30 mg / kg for 30 d five consecutive days per week). Animals were killed 35 days after the latest injection. Testes tissues sections were provided to investigate the histopathological changes. Serum testosterone, LH and FSH concentrations were measured by radioimmunoassay. Data were analyzed using of one-way ANOVA. Significance was set at p<0.05.<br> <strong>Results:</strong> A significant reduction was observed in diameter and weight of testes after DZN administration. Furthermore, DZN brought about significant reduction in sperm counts and spermatogenic, Leydig and Sertoli cells and a decrease in serum testosterone concentration. Histopathological examination of testes showed degenerative changes in seminiferous tubules (p<0.001). The levels of LH and FSH were increased in DZN groups compared to the control and sham groups (p<0.05).<br> <strong>Conclusion:</strong> DZN is a toxicant for mammals&rsquo; spermatogenic cells during the early spermatogenesis. Therefore, application of DZN should be limited to a designed program. Diazinon, Leydig cells, Testosterone, LH, FSH. 59 64 http://ijrm.ir/browse.php?a_code=A-10-549-212&slc_lang=en&sid=1 Esmail Fattahi e.fattahi@iauamol.ac.ir Yes Department of Biology, Ayatollah Amoli Branch, Islamic Azad University, Amol, Iran Kazem Parivar No Department of Biology, Research and Science Tehran branch, Islamic Azad University, Tehran, Iran Seyed Gholam Ali Jorsaraei No Fatemeh Zahra Infertility and health reproductive research center, Babol University of Medical sciences, Babol, Iran Ali Akbar moghadamnia No Department of Pharmacology, Babol University of Medical Sciences, Babol, Iran