In an attempt to characterize the endocrine profile of prostaglandin F2α (PGF2α) in relation to the female reproductive system, the compound (racemic form) was administered to hamsters and rats in various reproductive states. The prostaglandin terminated pregnancy when given once a day either subcutaneously (50 μg/hamster) or orally (1·5–2 mg/hamster) from Days 4 to 6 of pregnancy inclusive, or as a single subcutaneous injection (50 μg/animal) on Day 4. In the rat, higher (500 μg/injection) and more frequent (twice daily) s.c. injections were required to get even foetal resorption. Concomitant administration of progesterone (4 mg/animal) in either species protected pregnancy. Prostaglandin F2α terminated pregnancy without interfering with the Pontamine blue reaction, suggesting that its antifertility effects were not mediated by inhibition of implantation. In both hamsters and rats the prostaglandin markedly reduced the size of deciduomata which could be restored to normal by administration of progesterone. Prostaglandin F2α delayed passage of zygotes through the Fallopian tubes in a proportion of rats but failed to accelerate egg transport in rats and hamsters. Furthermore, it caused a marked histological degeneration of the corpora lutea and induced formation of a fresh set of corpora lutea in pseudopregnant, pregnant and pseudopregnant—hysterectomized hamsters. These deleterious effects of prostaglandin were accompanied, in hamsters, by the appearance of freshly ovulated tubal ova. Most of the endocrine effects of PGF2α observed in this study can be accounted for by its luteolytic property.