We examined the effects of the absence of the pineal gland on testicular maintenance and regression in adult male Mongolian gerbils. Pinealectomized and control gerbils were maintained in different photoperiods (0 L, 8 L, 14 L, or 24 L), and body weights and testicular maintenances were investigated. Testes sizes and body weights were measured every week for ten weeks. Body weight did not change during the course of the experiment in all photoperiods. In testes weights, there was no difference between pinealectomized and control groups in the 14 L photoperiod (p > 0.05), while there was a significant difference between pinealectomized and control groups in 0 L, 8 L, and 24 L photoperiods; in control groups there was a regression in testes weights. These results demonstrate that photoperiod does not affect the regulation of body weight, but it does affect the reproductive system of Mongolian gerbils. Thus, the pineal gland plays an important role. in the transduction of photoperiodic information.