We have investigated the LH response to LH releasing hormone (LH-RH) in female hypogonadal (hpg) mice in which the hypothalamus contains no LH-RH and the pituitary gland contains significantly less LH than in normal mice. Both the releasing action and the priming effect of LH-RH were not significantly different in hpg compared with normal mice. Raised plasma concentrations of oestradiol-17 beta reduced pituitary responsiveness to LH-RH in normal but not in hpg mice. These results show that in the mouse neither long-term exposure to normal levels of LH-RH nor a normal pituitary content of LH are necessary for either the releasing or the priming action of LH-RH.