Peripheral plasma progesterone concentrations in intact and hysterectomized pseudopregnant rabbits decreased from Day 14. The rate of decrease in intact, but not in hysterectomized, rabbits accelerated on Day 17 and was associated with an increase in PGF-2 alpha levels in the uterine venous plasma. There was no change in uterine PGE-2 output during pseudopregnancy. In pregnant rabbits, peripheral plasma progesterone concentrations remained high up to Day 30, and there was no increase in uterine venous plasma levels of PGF-2 alpha on Day 17, PGF-2 alpha levels were elevated in uterine venous plasma on Day 25 of pregnancy, but there was no concurrent decline in plasma progesterone concentrations. Uterine venous plasma levels in PGE-2 increased markedly after Day 11 of pregnancy and reached very high concentrations in some rabbits.