INFLUENCE OF THE PRESENCE OF RAMS ON THE LUTEINIZING HORMONE SURGE AFTER OESTRADIOL BENZOATE INJECTION IN OVARIECTOMIZED EWES

The continuous presence of rams reduces the duration of the sexual receptivity both in intact ewes during spontaneous oestrus (Parsons & Hunter, 1967) and in ovariectomized ewes after progesterone-oestrogen treatment (Fletcher & Lindsay, 1971). Moreover, the exposure of intact ewes to rams accelerates the surge of luteiniz-ing hormone (LH) resulting in earlier ovulation (Lindsay, Cogni\l=e'\,Pelletier & Signoret, 1975). Oestrogens appear to provide the primary stimulus for the LH surge in the ovariectomized ewe (Pelletier & Signoret, 1970; Scaramuzzi, Tillson, Thorneycroft & Caldwell, 1971). This present experiment was designed to investigate a possible ' male effect ' on the timing of the pituitary release of LH after progesterone-oestrogen treatment of the ovariectomized ewe. Thirty-seven mature Ile-de-France ewes, ovariectomized at least 1 month before the hormone injections, were used in the experiment. They were primed by five daily i.m. injections of 25 mg progesterone in oil, followed by a final injection of 10 mg progesterone on day 6. Forty-eight hours after the last injection of progesterone, the ewes were injected i.m. with 50 /¿g oestradiol benzoate in oil. Immediately after thi