The pineal organ in sockeye salmon and its relation with other epithalamic components has been studied. Histological and histochemlcal methods were used and the influence of experimental photoperiods on the histology and secretion in the juvenile pineals investigated. Photosensory and supporting ependymal cells have been identified. Lipofuchsin and melanin are absent in the pineal epithelium. Besides sensory fibres, efferent end-loops are present on the sensory and supporting cells. The dorsal pineal nerve tract is probably of a mixed type. Although the blood supply is profuse an endocrine or neuroendocrine activity is not indicated. It is proposed that the pineal organ in both juveniles and adults is photosensory and secretory. The apocrine secretory activity of the sensory and some supporting cells is probably associated with either the maintenance of constant chemical composition of the cerebrospinal fluid or supply of certain substances to the nervous tissue. The subcommissural organ is metabolically more active than the pineal. The secretion consists of glycogen, mucopolysaccharides, mucoproteins, glycoproteins and aldehyde-fuchsin positive granules. Chrome-alum positive granules are abundant in the subcommissural organ. Both the pineal organ and the subcommissural organ appear independent of light's influence on secretion and histology. It seems more plausible that apocrine secretory activity is controlled by some internal factors. It is suggested that the pineal organ might be of some advantage in the light dependent behaviour of this species in terms of intensity detection. Future work is necessary in this direction. / Science, Faculty of / Zoology, Department of / Graduate