The preliminary step in the involvement of gonadoliberin (GnRH) in the cellular mechanism for the secretion of the gonadotropic hormones LH and FSH, consists of a reversible binding of GnRH with specific site on plasma membranes of the pituitary gland. The parameters of this interaction have been determined in vitro with the aid of biologically active preparations of synthetic GnRH labelled with [3H] or [125I], and homogenates or preparations of plasma membranes, from the pituitary glands, of rat, sheep or beef. According to several authors, one or two types of sites are involved in GnRH-receptor binding. At 0-4 degrees C, the equilibrium association constants of the high affinity sites vary from 0.77 X 10(8) M-1 to 2.33 X 10(10) M-1. At 37 degrees C, we have obtained a value of 1.1 X 10(8) M-1 for this constant. Low affinity binding sites have been found in pituitaries and in other tissues; according to CLAYTON (1979), they may be structurally bound to enzymes which degrade GnRH. Other authors feel that a correlation may exist between the biological activity of agonistic analogues of GnRH and their capacity for binding to high affinity sites. Modulation of the binding of GnRH to low and high affinity sites by steroids and/or peptidases has been considered. The existence of intracellular receptors and internalisation of the GnRH-receptor complex have also been reported.