Injection of 150 micrograms 6-hydroxydopamine (6-OHDA) into the third ventricle of rats depleted the mean hypothalamic concentration of noradrenaline by 71% whereas the mean dopamine concentration was only reduced by an insignificant 7%. Isolated perfused pituitary glands taken from intact 6-OHDA-treated rats showed a markedly increased LH response to pulses of LHRH although there was no significant difference in the circulating levels of LH. Ovarian cyclicity was disrupted and the hypothalamic content of LHRH was significantly reduced. Hypothalamic synaptosomes prepared from intact 6-OHDA-treated animals consistently released less LHRH than did controls although the difference was not significant. Noradrenaline, dopamine and adrenaline did not significantly affect LHRH release from experimental or control synaptosome preparations. In ovariectomized rats 6-OHDA treatment did not inhibit the positive feedback effects of progesterone administration to oestrogen-primed animals, although the negative feedback effects of the priming oestrogen treatment was augmented. The results indicate that depletion of hypothalamic noradrenaline causes subtle changes in the endogenous release of LHRH and may alter the negative feedback effects of steroids on the hypothalamic-pituitary axis.