Heat stress can affect reproduction potential as an environmental factor. This Study was carried out to investigate the effects of rosemary extract on spermatogenesis and sexual hormones of laboratory mice under heat stress. 50 male mature mice were examined in five groups including a control group and four experimental groups [0, 100, 200 and 400 mg/kg of rosemary extract]. Samples were kept under heat stress four hours a day and received the extract doses for 30 days. At the end of the experiment, the amount of testosterone, LH, and FSH hormones plus the number of spermatogenic cells were measured. Obtained data were analyzed using the SPSS program. Heat stress in zero doses reduced testosterone, LH, and FSH significantly whereas rosemary extract increased testosterone and LH in 200 and 400 doses and FSH in 100,200, and 400 doses. Primary spermatocytes were decreased in zero doses significantly but increased significantly in other experimental groups [p<0.05]. In general, Heat stress reduces male sex hormones and spermatogenic cells but rosemary extract compensated this reduction dose-dependently and improved sexual potential under heat stress.  Full article