Plasma progestagens in cyclic, pregnant and post-partum mares.

The pattern of plasma progestagen levels during the oestrous cycle was similar to that previously reported except for lower peak levels. The lack of significant difference (p less than 0-01) between CPBA and RIA values suggests that progesterone itself is the major component during the oestrous cycle. Progesterone levels during the first and second post-parturient oestrous cycles were similar to those observed during the cycle of the non-pregnant mare. During pregnancy there were two peaks of plasma progestagens. The first, which occurred during the 3rd month, coincided with high levels of PMSG and was probably produced by the endometrial cups or the secondary CL. The second peak occurred in the 11th month and probably represented the secretion of placental progestagens. At least two unknown progesterone (?) metabolites were present which cross-reacted in the CPBA and RIA systems and probably contributed to the high progestagen values obtained in late pregnancy when chromatography was excluded from the assay procedures. Progestagen levels dropped precipitously after parturition and was probably due to placental separation from the endometrium. The higher progesterone levels in the newborn foal and other evidence suggests that this hormone is produced by the fetal gonads.