This study examined the in vivo distribution and metabolism of maternal progesterone (P4) in the rat uterus at Day 16 of pregnancy, i.e., the time of maximal P4 secretion, and at Day 22, one day prior to parturition. Arterial and uterine venous blood samples were collected at 20-min intervals from rats (n = 5 per group) infused with [3H]-P4 for 2 h. Placentas, fetuses, and uterine tissue (myometrium and decidua) were obtained just prior to the end of the infusion; and total tritium, [3H]-P4, and lipid-soluble and water-soluble metabolite concentrations were determined in all blood and tissue samples. Irreversible extraction of P4 by the uterus and its contents was 54.2 +/- 6.0% (mean +/- SEM) on Day 16, and this was at least maintained to Day 22 (64.7 +/- 7.4%). Because uterine blood flow increases dramatically over this period, the maintenance of high uterine P4 extraction is likely to have contributed partly to the 41% rise in the metabolic clearance rate of P4 between Day 16 (147 +/- 16 ml/min per kg) and Day 22 (207 +/- 13 ml/min per kg). Uterine tissue levels of [3H]-P4 exceeded (1.6-fold) those in arterial blood on Day 16, but this difference was not evident at Day 22. In contrast, the concentration of [3H]-P4 in the placenta was lower than that in arterial blood at Day 16 (66% lower) and Day 22 (25% lower), even though total tritium concentrations were similar at these sites. [3H]-P4 was also lower in fetal tissue compared with maternal arterial blood on both days of pregnancy.(ABSTRACT TRUNCATED AT 250 WORDS)