Departments of Medicine and Physiology, St Louis University School of Medicine, St Louis, Missouri 63104, and *Department of Poultry Science, University of Missouri, Columbia, Missouri 65201, U.S.A. (Received 27th January 1975) It is known that the oxytocin (vasotocin) response in the domestic hen varies during the period of a single egg cycle with a significant increase in sensitivity as the time of spontaneous oviposition approaches (Gilbert, 1971). In this respect, the avian shell gland is similar to the mammalian uterus. We have reported previously that intrauterine injection of prostaglandins (PG) or essential fatty acids (which may serve as prostaglandin precursors) can induce premature oviposition in the domestic fowl (Hertelendy et al., 1974) and the japanese quail (Hertelendy, 1972, 1974). In these studies premature oviposition was induced when there was a hard egg in the uterus, but several hours before the anticipated spontaneous oviposition. The present report is of the response