Initiation and maintenance of the reproductive axis in the human is contingent upon the pulsatile secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus. In man, tracking endogenous GnRH secretion relies on frequent peripheral sampling of three glycoprotein products of the gonadotrope, luteinizing hormone (LH), follicle-stimulating hormone (FSH) and free alpha-subunit (FAS). FAS is superior to LH as a surrogate marker of GnRH secretory dynamics in those circumstances where GnRH secretion is rapid or where the pituitary LH response is low. The pattern of GnRH secretion is constantly changing across development; from high levels during the neonatal period, through a period of quiescence in mid-childhood, followed by sleep-entrained reactivation of the reproductive axis at the onset of puberty, ultimately culminating in the adult pattern of pulsatile secretion which in the male is approximately every 2 h and in the female varies with the stage of the menstrual cycle. This episodic mode of secretion allows differential impact of feedback mechanisms on the GnRH neurons and facilitates the maintenance of the reproductive axis during periods of stress by allowing frequency and amplitude modulations.