The ultrastructure of the rostral pars distalis of adenohypophysis was examined in Oreochromis nilotica. The rostral pars distalis is mainly composed of adrenocorticotrophic cells, prolactin cells and one non-secretory cell type-stellate cells. The observations on the annual cycle of the structure of pituitary showed that the adrenocorticotrophic cells and prolactin cells become active during ovulation. It is suggested that ACTH secretion may facilitate ovulation and the prolactin cell activity is attached by changes in environmental temperature. In rostral pars distalis, only the prolactin cells receive direct innervation from type B neurosecretory ending. The steUate cells and type Ⅱ pituicytes play an important role in releasing and transfering the secretory granules from the secretory cells. Two means by which the secretory granules are released, exocytosis and diffusion from the secreting cells, are discussed.