Plasma progestogen levels in pregnant mares following administration of exogenous progesterone

Twelve pregnant mares were used in a switchback design with two groups of six mares each. On day 255 of gestation, group A was administered repository progesterone (250 mg, IM) and group B progesterone in sesame oil (250 mg, IM). Jugular vein plasma was taken 15 min pre-injection and post-injection at 30 min, 1, 3, 6, 9, 12, and 24 hours. Additional samples were taken on days 2, 3, 4, 6, and 8. Both groups were then assigned to the opposite treatment on day 270 of gestation. Concentrations of plasma progestogens after injection with progesterone in sesame oil were not different (P>.05) from values obtained using repository progesterone through the 8 days studied. Mean progestogen concentrations after injection of progesterone in sesame oil were, however, consistently higher through 48 hr than those given repository progesterone. Concentrations of progestogens were not different (P>.05) from pre-injection concentrations by day 6 post-injection using either vehicle.