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The concentrations of a number of reproductive hormones found in the peripheral plasma of the cow around the time of oestrus have been measured by numerous authors: plasma concentrations of progesterone throughout the oestrous cycle (Stabenfeldt, Ewing & McDonald, 1969), LH (Schams & Karg, 1969), oestrogen and progesterone (Henricks, Dickey & Hill, 1971) and LH and progesterone (Henricks, Dickey & Niswender, 1970). These and other reports have paid little attention to the precise onset and duration of oestrus, behaviour being checked at various intervals (usually two or three times a day) before the expected onset. To obtain a more precise pattern of hormone concentrations around oestrus, we have measured simultaneously LH, oestradiol-17\g=b\and progesterone in samples of plasma taken at 2-hr intervals from cows over a period of 5 days. The cows were maintained with a bull, to allow continuous observation of sexual behaviour. By such observation, we have attempted to define the true onset ofoestrus, to dissociate it from preliminary pro-oestrous behaviour and to correlate the behavioural manifestations of oestrus with the internal hormonal parameters. This preliminary study was intended to establish hormonal criteria in the normal cyclic cow for later comparison with animals undergoing treatment for synchronization of oestrus, when administration of exogenous hormones might modify the endogenous hormone balance, and when a decrease in fertility often results. Blood samples were collected from the jugular vein of four cyclic, multi¬ parous Friesian cows, at intervals of 2 hr for a period of 5 days, starting on Day" 18 of the cycle (day of oestrus = Day 0). Daily blood collections were made for several days before this, and until Day 10 of the following cycle. Plasma was separated by centrifugation and stored at

PERIPHERAL PLASMA CONCENTRATIONS OF PROGESTERONE, OESTRADIOL-17 AND LUTEINIZING HORMONE AROUND OESTRUS IN THE COW