Plasma progesterone concentrations derived from the administration of exogenous progesterone to ovariectomized mares

Six ovariectomized mares were divided into 3 groups to determine the effects of exogenous progesterone in oil and repositol progesterone on plasma progesterone concentrations. Progesterone in oil was administered in 7 daily injections in Exp. I. Progesterone concentrations were not maintained greater than 1.0 ng/ml for 24 h with 50 mg/day. However, they remained greater than 1.0 ng/ml during the last 4 days of 100 mg/day and greater than 1.5 ng/ml throughout the injection sequence of 200 mg/day. Repositol progesterone was administered on Days 1 and 7 in Exp. II. At 500 mg, progesterone concentrations peaked in 6 h but returned to near 1.0 ng/ml in 2 days. At 1000 mg and 2000 mg, plasma progesterone was maintained at approximately 2.0 and 4.0 ng/ml respectively for 7 days after injection on Day 1 and was 1.5 and 3.5 ng/ml respectively, 11 days after injection on Day 7. An indication of a cumulative effect on plasma progesterone was observed following repeated dosages of both progesterone in oil and repositol progesterone.