In Fishes the endocrine systems involved in calcium homeostasis consists of corpuscles of Stannius (that produce stanniocalcin), ultimobranchial gland (that produce calcitonin) and prolactin cells (that produces prolactin). There is seasonal variation in the activity of both corpuscles of Stannius and ultimobranchial gland with respect to reproductive cycle. The glands exhibit signs of hyperactivity during prespawning and spawning phase however their reduced activity during post spawning phase and almost inactivity were noted during spent phase. Hyperactivity of CS and UBG is due to elevated calcium level that accompanied ovarian maturation. The prolactin cells exhibit hyperactivity during prespawning and spawning period while their diminished activity was noted after spawning when serum calcium level abruptly falls. This increased activity of prolactin cells was due to increased estradiol secretion. The present book describes the morphology, seasonal cytological changes in the Ovary, Ultimobranchial gland, Corpuscles of Stannius and pituitary Prolactin cells of female Mastacembelus armatus in relation to reproductive cycle and serum calcium level.