Plasma LH levels in the mare during the oestrous cycle

Plasma levels of LH were determined by heterologous radioimmunoassay utilizing highly purified equine LH as standard. Samples were taken regularly from eleven mares for twenty-six oestrous cycles over a period of 10 weeks. The mean cycle length was 20-5 +/- 3-1 (S.D.) days, and ovulation occurred on average 4-3 +/- 1-6 (S.D.) days from the time heat was first detected. Levels of LH were persistently low from Days 5 to 16 of the cycle (ovulation = Day 0). They then increased slowly over a number of days and continued to rise beyond the levels observed at any time during the immediate preovulatory period or the day of ovulation. A significant decrease from peak levels was not observed until the 3rd day after ovaulation, from which time levels continued to decline toward dioestrous values by an apparent first-order decay process with a half-life of 1-8 days. The pattern of plasma LH in the mare differs from that reported for other species and it is suggested that persistance of high concentrations of LH results from a long half-life of the endogenous LH. This in turn may be responsible for the relatively large number of second ovulations detected in many oestrous cycles.