Prostaglandin (PG) levels increase dramatically just before parturition in the rat. Coincident with this dramatic increase in uterine PGs is a precipitous decrease in plasma progesterone and enhanced plasma estradiol levels. The purpose of the present study was to mimic the progesterone withdrawal phenomenon in the presence and absence of estradiol in ovariectomized pregnant rats and determine the effects on uterine PGF, PGE, thromboxane B2 (TxB2) and 6-keto-PGF1a (6KF) levels. Rats were ovariectomized on day 16 of pregnancy and silastic inserts containing progesterone and estradiol placed s.c. in two groups of rats (I and II) while the third group (III) received progesterone only. On day 19 of pregnancy progesterone was withdrawn from groups II and III and rats sacrificed 0, 6, 12 and 24 hours later. Uterine tissue was assayed for PGs by radioimmunoassay. Progesterone withdrawal in the absence of estradiol (III) administration significantly (p less than .05) elevated PGE, TxB2 and 6KF, but not PGF, at the 24 hour period compared to controls (I). When progesterone was withdrawn in the presence of exogenously administered estradiol (II) only PGF showed enhancement (p less than .05) over III at the 24 hour period thus indicating a specific effect of estradiol on the PGF metabolic pathway. In conclusion, these data indicate that: (1) progesterone withdrawal is a potent stimulus for uterine PG production and is probably a major contributor to the augmented uterine PGE, TxB2 and 6KF levels at term in the pregnant rat; and (2) progesterone withdrawal in the presence of exogenously administered estradiol enhances uterine PGF production thus indicating a specific effect of estradiol on PGF production.