The surface area and luminal structures of the pineal sac were studied during the prespawning, spawning, and postspawning phases of control and gonadectomized fish, Mystus vittatus, throughout the year. From the later half of prespawning to the first half of postspawning phases the values for the surface area were lower in the gonadectomized group whereas from the later half of the postspawning to the first half of prespawning phases the values were higher than in the control group. The acidophilic cell bodies were visible in the first half of prespawning and at the end of spawning phases as well as during the postspawning phase. In the control group sulfated acid mucopolysaccharides and mucoid basophils with gamma substances were faintly stained during the early months of the reproductive cycle in contrast to the experimental group; but in the later months these substances were strongly stained in both groups except in the gonadectomized group at the end of postspawning phase. The pineal lumen was narrow during the pre- and postspawning phases and wider in the spawning phase of the gonadectomized group in comparison to control; this may be related to the flow of cerebrospinal fluid. Thus, the morphohistochemical changes in the pineal organ of fish were evident in both groups during the prespawning, spawning, and postspawning phases.