Although there have been reports on the effects of adrenal and pineal gland secretions on the hypothalamus and the pituitary, the maintenance of rhythmicity by these glands has not been reported so far. The present data deals with the alterations in the periodicity of hypothalamo-pituitary axis after the surgical removal of either adrenal, gonad or pineal in adult male rats. The data indicated an alteration in the 24 hour pattern of hypothalamic content of LHRH after adrenalectomy, castration or pinealectomy. In adrenalectomy group, the in vitro responsiveness of pituitary decreased at 0600 h and 100 h as compared to the intact rats, resulting in a low amplitude of LH circadian rhythm. The pituitaries of pinealectomized rats showed increased sensitivity at 1000 h and 1800 h which resulted in two peak LH concentration at those time intervals. Further, pinealectomized rats showed a selective five fold increase over 24 hr in serum FSH concentration as compared to intact rats suggesting dissociation in the release of FSH and LH.