The central part of menstrual cyclicity is the arcuate nucleus within the mediobasal hypothalamus, which is shown to be the origin of GnRH. This neuronal oscillator is influenced by many endogenous substances and its periodically activity is modulated by them. The endogenous opioids are such substances. They are widely distributed within the organism and are endogenous ligands of opiate receptors. The endogenous opioids participate in many somatic functions such as regulation of ovarian cyclicity. Furthermore they are causal involved in hypothalamic amenorrhea.