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Instituto de Neurobiologia, Serrano 665, Buenos Aires, Argentina

(Received 9 June 1977)

Changes in the levels of catecholamines in the hypothalamus during the oestrous cycle have been reported by Donoso, Stefano & Biscardi (1966) and Stefano & Donoso (1967). Shaar & Clements (1974) found that lower concentrations of dopamine and noradrenaline than those found in the hypothalamus can inhibit the release of prolactin from the rat pituitary gland. We have studied the levels of noradrenaline and dopamine in the median eminence around the time of the spontaneous preovulatory release of hormones.

Adult female Holtzman rats showing at least three consecutive 4 day cycles, were killed by decapitation at 09.30, 13.00 and 17.30 h on the days of dioestrus I and pro-oestrus. After decapitation, blood was collected into centrifuge tubes and the brains were immediately removed and frozen on solid CO₂. Serum was separated by centrifugation and stored at –20 °C