Intrasexual as well as intersexual dimorphisms were found in the prosencephalon and mesencephalon of adult Oncorhynchus nerka (red/sockeye salmon). These dimorphisms are concerned with the position of the preoptic nucleus, nucleus lateralis tuberis, habenula, third ventricle, tectal ventricles, preoptic recess, recessus lateralis, horizontal commissure, posterior commissure, and toral commissure. The intrasexual dimorphism was characterized by either a rostral ("r"-pattern) or a caudal ("c"-pattern) position of the preoptic region as well as varying locations of other structures within the prosencephalon. As compared to "c"-pattern fish, the preoptic nucleus and nucleus lateralis tuberis were located more rostral, and the habenula was positioned further caudal, in "r"-type animals. The intersexual dimorphism was also characterized by different positions of the structures listed above. With the exception of the preoptic nucleus, all of these were located further rostral in "r"-pattern females than in type "r" males. In "c"-pattern females, they were positioned further caudal than in type "c" males. The number of neurons in the parvocellular and in the magnocellular portion of the preoptic region differed in the two genders with respect to "r"- as well as "c"-pattern fish. Males had more neurons than females in both the magno- and the parvocellular subdivisions of the preoptic region. In "r"- and "c"-pattern fish, the average size of magnocellular preoptic neurons was larger in females than in males. The observed intersexual variations may reflect gender-specific differences in the control of the pituitary. Functional correlates of intrasexual dimorphism are obscure.