Serotonin or melatonin was infused into the 3rd cerebral ventricle (3rd V) or into the medial basal hypothalamus (MBH) of both intact ewes and ewes with lesions of the anterior hypothalamic area (AHA). Prolongation of the estrous cycle and delay of ovulation were obtained after infusions of melatonin or serotonin in both the intact and lesioned ewes. Statistically, serotonin proved to be more effective in this respect than melatonin in the intact ewes. The infusions also blocked the preovulatory peak of LH. Furthermore, the infusion of serotonin blocked estrous behavior, while that of melatonin did not affect this function. The effect of indoleamie infusions in the lesioned ewes, in which the inhibitory action of the AHA over the transmission of gonadotrophic releasing hormones (G-RH) had been eliminated, seems to indicate that the inhibitory action of indoleamines on the release of these neurohormones is being displayed at the level of the MBH.

Doma\l=n'\ski& Kochman (1968) showed that sheep were more responsive to infusion of purified hypothalamic extract into the adenohypophysis in late anoestrus than in mid-anoestrus. Natural (Schally, Redding, Matsuo & Arimura, 1972) and synthetic (Matsuo, Arimura, Nair & Schally, 1971) LH-RH have been used to induce ovulation in many species, including sheep (Reeves et al., 1972). Administration of a single intravenous injection of 150 \g=m\gsynthetic LH-RH to seasonally anoestrous ewes induces LH release in all injected animals, but the LH peak is much smaller than that observed at oestrus and luteal function, assessed by peripheral progesterone levels, is absent in most animals (Crighton, Foster, Haresign & Scott, 1975). When the LH-RH was given as five injections the LH peak resembled that at natural ovulation but plasma progesterone levels were still low. Shareha, Ward & Birchall (1976) gave much smaller doses of LH-RH (25 \g=m\g/day in continuous infusions for 2 days) to anoestrous ewes but the results were again unsatisfactory.Jeffcoate, Greenwood & Holland (1974) showed that the rate of disappearance of LH-RH from the circulation in normal human subjects is represented as a double exponential with half-lives of the two components being 5-3 and 27-4 min. Kochman, Kerdelhué, Zor & Jutisz (1975) reported that degradation of the hormone, caused by polypeptidases in the hypothalamic and pituitary tissues, occurs within 30 min. We therefore investigated the effects of prolonged infusions of small doses of

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