The principal physiological site of action of GnRH is on pituitary gonadotrophs. Although binding sites for this peptide are described in the gonads, placenta, breast and brain, their precise physiological/pharmacological relevance remains to be elucidated. The pituitary action of GnRH can be divided conveniently into an immediate release of LH and FSH within minutes, the synthesis of LH and FSH over a matter of hours (intermediate action), and long-term morphologic changes lasting several days. Most, if not all, of these actions are initiated following interaction with high-affinity (Kd = 0.5 nM) stereospecific receptors on gonadotrophs. GnRH receptor concentration is negatively regulated by testosterone and progesterone, and positively regulated by oestradiol in vivo as well as by the ligand itself. In-vivo and in-vitro GnRH regulates its own receptors depending on its pulsatile or continuous secretion which increase or decrease numbers (up-or down-regulation), respectively. In-vivo change in GnRH receptors probably reflect the extent or pituitary exposure to endogenous GnRH and is an indirect index of hypothalamic GnRH secretion. Up-regulation is not ligand specific since other activators of LH release e.g. depolarization, ionophores, and cAMP derivatives, are also effective in vitro. Down-regulation is specific to the ligand and contributes to ‘desensitization’ which also includes disruption of the signal transduction mechanism (uncoupling of receptors), and depletion of cellular LH stores. GnRH receptor occupation activates membrane phosphoupase C to hydrolyse polyphosphoinositides with the formation of inositol-l-4-5-trisphosphate and diacylglycerol which activates protein kinase C. Both intermediates act synergistically on LH release, but their role in intermediate and long-term actions of GnRH has not been determined. The initial rapid release of LH depends on the mobilization of intracellular calcium whereas sustained release over minutes depends on the entry of extracellular calcium through opening of receptor-activated channels. Early studies suggested that GnRH stimulates LH glycosylation and more recently direct evidence for stimulation of LH subunit protein biosynthesis has been shown in vivo using cDNA–RNA hybridization assays to measure cytoplasmic mRNA. GnRH is required for the post-orchidectomy increase in α and LH-β mRNA, and in GnRH-deficient animals the peptide directly stimulates subunit mRNA levels implying an action on transcription of specific genes. Desensitization of gonadotrophs with GnRH in vivo involves marked reduction in LH-β sub-unit mRNA, while mRNA levels are increased. Levels of LH-β mRNA, which are closely correlated with pituitary LH content, are the rate-limiting step in the biosynthesis of LH. A nuclear action of cellular mediators of GnRH action is highly likely.