Objective:To determine the differences in the activity of pituitary prolactin cells between male and female fresh water teleostMastacembelus armatus(M. armatus) during their reproductive cycle.Methods:Fishes were sampled every month throughout the year.They were dissected, gonads weighed for the determination ofGSI.Blood samples were collected for estimation of plasma calcium,17β-estradiol of females and testosterone of males and prolactin.Nuclear diameter ofPRL cells, diameter of oocyte and testicular lobule was measured by image analyzer microscope.An experiment was also conducted in which female and male fishes were injected with17β-estradiol and17 alpha-methyltestosterone respectively and the blood samples were analyzed for plasma calcium, prolactin and sex steroids levels.Results:Negligible change in plasma calcium and prolactin level and little variation in nuclear diameter ofPRL cells were recorded throughout the year in relation to testicular cycle as well as after17 alpha-methyltestosterone administration in male.Variations in level of plasma calcium and prolactin and a large range of difference in nuclear diameter ofPRL cells were noted during ovarian cycle as well as after17β-estradiol administration in female.Conclusion:Difference in the activity ofPRL cells was noted between male and femaleM. armatus.It act as hypercalcemic factor in females whose level increase along with maturation of ovary with the influence of increasing level of17β-estradiol from ovarian follicles to fulfill the increased demand of calcium for vitellogenesis and thus directly effecting reproduction.No such role of prolactin was observed for male fishes.