The rates of metabolism of progesterone and 20α-dihydroprogesterone (20α-diHP) in sheep have been measured during and after the infusion of tracer amounts of [3H]progesterone. There were significant differences in the blood concentration of [3H]progesterone between experiments, but these were not attributable to the stage of gestation or to the difference between pregnant and non-pregnant animals. The mean (±S.E.M.) metabolic clearance rate of progesterone was 3·277 ± 0·239 litres blood/min.
The simplest model of the distribution of progesterone was one containing two pools, V1[P] and V2[P], where [P] is the blood concentration of progesterone, and in 23 experiments on 7 sheep the mean pool dimensions were 7·8[P] μg and 70[P] μg, respectively. This model was developed to include the formation of 20α-diHP from progesterone. Progesterone appeared to be the major source of 20α-diHP, though this did not seem to be an obligatory metabolite. The parameters obtained gave comparably low residual deviations for both labelled steroids and were consistent with other observations made on progesterone clearance.