Long-term ovariectomized rats received a single injection of 20 micrograms oestradiol benzoate (OB) which reduced the serum levels of LH for at least 3 days. The inhibitory effects were accompanied by time-dependent alterations of noradrenaline and dopamine turnover rates in the mediobasal hypothalamus (MBH) and the preoptic-anterior hypothalamic brain area (POAH). Oestradiol markedly interfered with the time-dependent variations of noradrenaline and dopamine turnover seen in the MBH of untreated ovariectomized animals during daylight hours. In the POAH the turnover rate of noradrenaline decreased 2 days after priming with OB and then increased in the afternoon of day 3. The increase of noradrenaline turnover in the POAH was accompanied by a low afternoon turnover rate of dopamine in the MBH and by an increased sensitivity of the LH secretory system to progesterone. Dopamine and noradrenaline turnover involve a time element. While the negative feedback actions of oestradiol do not seem to be associated with changes in dopamine or noradrenaline turnover, the results support the view that the induction of LH afternoon surges depends upon an increase of stimulatory noradrenergic inputs to the POAH and a decrease of inhibitory dopaminergic inputs in the MBH.