Aim: This study was aimed to examine the effect of testosterone on the pineal gland by light microscopy.

Material and method: For this purpose 36 male Wistar rats were used. Animals were divided into three groups. Group I and II were designated as sham-orchidectomized (Control) and orchidectomized, respectively. They received sesame oil (0.1 ml subcutaneously) alone. The rats in Group III were orchidectomized and daily injected with Testosterone propionat (0.5 mg/0.1 ml sesame oil per day s.c) for 1 months. At the end, all animals were killed by vascular perfusion. The pineal glands of rats were removed, then processed for light microscopy.

Results: It was observed that extensiveness of pinealocytes in the structure of pineal gland and enlargement of pinealocyte cell nuclei after orchidectomy. Some nuclei of pinealocytes contain double nucleoli were seen. Furthermore, lipid droplets were increased in the structure of pineal gland. In the testosterone injected group, histological changes, which indicate increased cell activation, occuring after orchidectomy were dissappeared.

Conclusion: Increased of pineal gland activity was seen after orchidectomy and this increase was suppressed following the injection of Testosteron hormone.