Abstract: The purpose of research work was to examine the effects of different photoperiods (24L:0D; 14L:10D; 8L:16D and OL:24D) on testes and accessory sex organs in mature male Mongolian gerbils (Meriones unguiculatus). 14L−born and raised animals with functional testes were held under four different photoperiods for ten weeks. Testicular size were measured every week for ten weeks. At sacrifice, testes of 14L animals were large and showed full spermatogenesis and possessed tall columnar epithelial cells and secretion in the lumen of seminal vesicles. Transfer to 24L, 8L, or 0L resulted in a decrease in testes size; the seminal vesicles had an epithelium with low columnar or cuboidal cells and contained a reduced amount of secretion in lumen. The histological appearances of testes were similar in 24L, 8L and 0L. Therefore, very long and short photoperiods have a strong effect on reproductive system in this species.