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Pro-oestrous surge in plasma progesterone in the cow.

A preovulatory rise in peripheral plasma progesterone concentration has been reported in women, monkeys, rats, mice, hamsters, guinea-pigs and rabbits (Kazama & Hansel, 1970). In the cow, a preovulatory rise in plasma progesterone levels was not found when blood samples were collected once daily (Shemesh, Lindner & Ayalon, 1971) every 2 to 3 hr for 10 to 12 hr, starting with the onset of oestrus (Henricks, Dickey & Niswender, 1970), or every 6 hr from the onset of oestrus until ovulation (Kazama & Hansel, 1970). The latter authors noted the possibility that a transient rise and fall in plasma progesterone concentration may have been missed due to infrequency of sampling during the pro-oestrous period. The present investigation was undertaken to examine this possibility. 48-hr