Summary. When [3H]progesterone is infused intravenously into ewes, blood 20α-dihydroprogesterone (20α-diHP) becomes labelled and the changes in [3H]20α-diHP activity with time are clearly related to that of [3H]progesterone. Concentrations of 20α-diHP in blood have now been estimated for these experiments. During the infusion, the specific radioactivity of 20α-diHP at steady state was only 53% of the specific radioactivity of progesterone, indicating that 20α-diHP was produced other than by C-20 reduction of secreted progesterone. The change in blood concentration of 20α-diHP during pregnancy in ewes suggests that the placenta is its source.
[3H]20α-dihydroprogesterone was infused into pregnant ewes and the specific radioactivity of 20α-diHP measured during and after infusion. Together with information from earlier experiments when [3H]progesterone was infused, there is now sufficient data to estimate, without constraint, the parameters of a four-pool model describing the distribution and metabolism of progesterone and 20α-dyhydro-progesterone in sheep.