The current study was designed to illustrate the histological variations of pituitary gland and
interrenal tissue of mullet fish (Mugil cephalus) reared in fresh and marine waters. In this
study, the pituitary gland of Mugil cephalus consisted of adenohypophysis and
neurohypophysis. Three regions were observed in adenohypophysis, rostral pars distalis
(RPD), proximal pars distalis (PPD) and pars intermedia (PI). There was no adrenal gland in
Mugil cephalus resemble mammals but interrenal tissue and chromaffin cells were located
in the wall of posterior cardinal vein in the head kidney. Light microscope and statically
studies showed an increase in the square surface area of RPD including the surface areas of
the prolactin (PRL) cells adrenocorticotrophs (ACTH cells) in adenohypophysis and
interrenal tissue in the head kidney of the freshwater fish than marine fish. These results
indicate the important role of prolactin, adrenocorticotropin and cortisol hormones in
osmoregulation in the freshwater Mugil cephalus and help in stress and habitat adaptation.