PROGESTERONE CONCENTRATIONS IN THE PERIPHERAL PLASMA OF THE MARE DURING THE OESTROUS CYCLE

Abstract

Progesterone levels in the peripheral blood of non-pregnant mares have been measured by Short (1959); progesterone was detectable only when there was a fully developed corpus luteum present in the ovaries. On the other hand, Short (1964) has shown that progesterone secretion into the ovarian vein, negligible at oestrus, increases very significantly within the first 24–36 hr. after ovulation. More sensitive methods for the measurement of progesterone concentration in peripheral plasma have since been developed. Peripheral plasma progesterone concentrations have been measured throughout the menstrual cycle of women (Neill, Johansson, Datta & Knobil, 1967a) and monkeys (Neill, Johansson & Knobil, 1967b) and throughout the oestrous cycle of the ewe (Thorburn, Bassett & Smith, 1969) by competitive protein-binding techniques and in cows, pigs and sheep by gas—liquid chromatography (Stabenfeldt, Ewing, Patton & McDonald, 1969). The present study using a protein-binding assay method was undertaken in an attempt to demonstrate