The remarkable morphinic capacities of endogenous opioid peptides give these agents particular and interesting properties. They can be found on different locations throughout the human body, where they bind to specific receptors. By means of various agonists and antagonists it was shown that endogenous opioids possibly play a prominent role in the control of hypophyseal secretions. The menstrual cycle is thought to represent one of the most important phenomena in which these agents are involved. In the presence of a steroid-sufficient environment (that is to say, in the luteal phase) 0-endorphin exerts a tonic inhibition on the secretion of GnRH and thereby on the LH-release. Moreover, the regulation of prolactin is probably under the control of endorphins interfering via dopamine (PIF). It becomes possible to explain a number of menstrual dysfunctions in this manner. The increased plasma concentrations of 0-endorphin, which are found after physical exercise, give rise to speculations as to their involvement in the frequently appearing menstrual irregularity in women athletes. The hypothesis proposing prolactin as being entirely responsible for these phenomena is unconvincing. We believe that endogenous opioid peptides could have a possible inhibitory effect on the GnRH-LH-axis. At this moment, methodological inaccuracies and differences of various studies make it very difficult to arrive at any firm conclusion.