Prostaglandin E2 (PGE2), but not oxytocin, treatment to induce parturition brought about an increase (p < 0.001) in maternal serum levels of total estrogens (conjugated and nonconjugated estriol). There was no increase in nonconjugated estriol. To explore if the changes were related to an altered enterohepatic circulation, cholic acid assay in maternal serum was used. There were no changes in cholic acid which parallelled those in total estrogens. It is concluded that PGE2 does not appear to affect enterohepatic circulation of steroids, and it is suggested that the increase in total estrogens may be caused by changes in the kidney excretion of estriol.