Using quantitative microfluorimetry in combination with tyrosine hydroxylase inhibition (H44/68) the concentration and turnover of noradrenaline (NA) and dopamine (DA) was studied in the subependymal layer (SEL) and the medial (MPZ) and lateral palisade zone (LPZ) of the rat median eminence during the 4- and 5- day vaginal estrous cycle. Significant cyclic variations were only found in SEL and LPZ. The NA turnover in SEL was high on proestrous and low on all other days of the 4-day estrous cycle, whereas in the 5-day estrous cycle the NA turnover in SEL started to increase already on the second day of diestrous to reach a peak in the afternoon of proestrous. At that time also the NA concentrations in SEL were increased, although significantly only in the 5-day cyclic rats. The DA turnover in LPZ was low on proestrous and high on all other days in both 4-and 5-day cyclic rats. Apart from the median eminence cyclic variations in catecholamine metabolism were only found in the medial preoptic area, where NA turnover was high on proestrous and low on estrous-diestrous. The present findings give further support for the existence of a facilitatory noradrenergic and inhibitory dopaminergic mechanism in the control of gonadotrophin release. Furthermore, it is suggested that an acceleration of reticulo-hypothalamic NA turnover precedes the retardation of tubero-infundibular DA turnover found on proestrous and that the time lag between initial NA activation and subsequent DA inactivation is longer in the 5-than in the 4-day estrous cycle.