It is generally accepted that the sexual cycle in mammals is accomplished by the systematic coupling of components of the hypothalamo-hypophyseo-gonadal system. However, how these components are related to each other to develop the sexual cyclicity has not yet been clearly established. A gonadotropin releasing factor (GnRF), which has been extracted and purified from porcine hypothalamus and whose molecular structure was determined in 1971, has been synthesized in many laboratories. The synthetic GnRF, named LRF or LH-RH, has gonadotropin releasing activity for many species of mammals, including humans. The debut of the synthetic LRF has given us a pure material and a direct means of stimulating, relatively easily, the gonadotroph cells in the anterior pituitary glands. In the present study, variations in the gonadotropin releasing activity of the pituitary gland in relation to synthetic LRF was investigated throughout the sexual cycle of rats. The relationship between pituitary sensitivity and levels of steroid hormones in the blood is discussed.