To investigate the histological structure and seasonal variation of the pituitary from Trionyx sinensis,the histological structure of T.sinensis pituitary in September and January was observed under light microscope and electron microscope.HE,PAF-AB-PAS-OG staining and immunohistochemical study were undertaken using different dyes or antisera.From the anatomical point of view,the pituitary of T.sinensis was composed of two lobes:the adenohypophysis and the neurohypophysis.It was functionally linked to the hypothalamus by the pituitary stalk.Adenohypophysis was composed by pars tuberalis and pars distalis,but pars intermedia was absent in the T.sinensis.Pars distalis could be divided into head lobe and tail lobe,and the former was larger than the latter.The neurohypophysis was small and fingerlike extending into the pars distalis.Different cells of adenohypophysis were divided into three cells of eosinocyte,basicytes and chromophobe cells.The shape and distribution of those kinds were different obviously.The eosinocytes were mainly located in the central region of pars distalis while the basicyte in the surrounding area,eosinocytes were generally smaller than basicyte and possess a round and sidelined nucleus,the basicyte were bigger and possess a central nucleus; the chromophobe cells and adrenocorticotropic cells were densely distributed in the pars tuberalis and sporadically distributed in the pars distalis.The quantity of somatotrophs and gonadotrophs in breeding season was higher than that in winter while other cells did not change significantly.Under electron microscope,the cells in adenohypophysis were divided into six categories of somatotrophs,gonadotrophs,thyrotrophsprolactin cells,adrenocorticotropic cells,adrenocorticotropic cells and chromophobe cells according to the cell size,shape of nucleolus and secretory granules in the cytoplasm.The quantity of secretory granules in cytoplasm of somatotrophs and gonadotrophs changed significantly in summer and winter.