The development of melatonin receptors in the rat pituitary and median eminence was studied using [125I]melatonin as a ligand. The specific binding was detected in pituitaries of 20-day-old fetuses already. The affinity of the receptor to the ligand (Kd) was in the range 63–133 pM and it did not change significantly during development. The pituitary concentration of [125I]melatonin binding sites was highest in 20-day-old fetuses (Bmax = 31 fmol/mg protein) and then it gradually decreased in the course of postnatal development, until it reached 10% of that value in 29-day-old males. In contrast, the concentration of melatonin receptors in median eminence did not change markedly in the course of development and it was about 15 fmol/mg protein. The marked decrease in the number of the pituitary receptors may be the cause of the reported developmental loss of the melatonin inhibitory effect on LHRH-induced LH release from anterior pituitar