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as a prolongation (Hunter, 1962) or precipitated onset (Smith, 1962) of the seasonal period of sexual rest. It has been suggested that the ovarian quiescence noted in underfed animals may be the result of pseudohypophysectomy (Mulinos & Pomerantz, 1940; Lamming, 1966; Leathem, 1966). An acute release of LH at oestrus in ewes has been demonstrated by Nis¬ wender, Roche, Foster & Midgley, 1968; Goding, Catt, Brown, Kaltenbach, Cumming & Mole, 1969; Wheatley & Radford, 1969) and it is possible that changes in the pattern of LH release may indicate functional changes in the pituitary of ewes which become anoestrous after periods of inanition. This phenomenon was studied in an attempt to elucidate the mechanism involved in a premature cessation of breeding amongst ewes previously subjected to underfeeding. During August, September and October 1970 and 1971 and in October 1972, blood samples were obtained at oestrus from eighty-seven Merino ewes that had been fed either 100% (high plane) or 50% (low plane) of their esti¬ mated daily nutrient requirements during the preceding 84-day autumn lactation period. The experimental rations, changes in body mass and oestrous activity of the ewes have been described elsewhere (Lishman, Stielau & Botha, 1974). Jugular blood samples were obtained (indwelling cannulae) from at least five ewes, on each plane of nutrition, which exhibited oestrus after the com¬ mencement of a sampling period (i.e. August, September or October 1970 and 1971). The blood samples were taken at 2-hr (1970) or 1-hr (1971), intervals, commencing as soon as a ewe stood for service and sampling continued for up to 36 hr. During October 1972, the first blood was drawn approximately 12 hr before the expected onset of oestrus in thirteen ewes. Sampling was repeated at 4-hr intervals until mating commenced and thereafter at hourly intervals for 24 hr. A total of ninety-two oestrous periods were studied. In order to detect oestrus, vasectomized rams were placed with the ewes at intervals of not more than 2 hr, and the rams were frequently interchanged between various groups of ewes. As soon as a ewe permitted service, she was immediately removed from the presence of the ram. The double antibody radioimmunoassay for ovine LH, as described by

THE RELEASE OF LH AT OESTRUS IN EWES ON TWO PLANES OF NUTRITION DURING LACTATION