Unité de Neurobiologie (U-109) de l'I.N.S.E.R.M. 2ter, Rued'Alésia 75014, Paris, France (Received 24 March 1975) We have shown previously (Hery, Laplante & Kordon, 1975) that blockade of serotonin (5-HT) biosynthesis by p-chlorophenylalanine (p-CPA) specifically inhibits the circadian release of luteinizing hormone (LH) observed in ovariectomized, oestradiol-implanted rats. Since changes in plasma LH levels can often be correlated with variations in the pituitary sensitivity to luteinizing hormone releasing hormone (LH-RH) (Cooper, Fawcett & McCann, 1974), it was important to check whether 5-HT depletion was able to affect directly the response of the pituitary. In addition, since adrenal progestins have been shown to affect gonadotrophin secretion in ovariectomized animals (Lawton, 1972), and in view of the effects of p-CPA on corticotrophin release (Scapagnini, Moberg, Van Loon, De Groot & Ganong, 1971), it was also relevant to make sure that the gonadotrophin changes we described after treatment with that drug did not involve