Pseudopregnancy was induced in 26 day old female rats by giving 30 IU of PMS followed 56 hours later with 5 IU of hCG. Day 1 of pseudopregnancy was considered established 72 hours after PMS administration. Pseudopregnancy lasted 14 to 15 days. Ovarian weights increased for 4 to 8 times due to treatment. Histological examiniation of the corpora lutea (CL) of pseudopregnancy suggested luteolysis began on day 6 and extended to day 8. A "new" crop of CL appeared on day 9 suggesting the duration of pseudopregnancy was supported by more than one generation of CL or by CL maturing at different rates. Twice daily administration of 1 mg PGF2alpha on days 5 to 8 prevented the appearance of the "new" CL on day 9, and increased signs of luteolysis in the initially formed CL. Lower doses (0.01 and 0.1 mg, b.i.d., X 4 days) delayed the appearance of "new" CL until day 10. Blood samples withdrawn between 0930 and 1100 hours were analysed for FSH, LH and prolactin. Animals treated with 0.01 and 0.1 mg of PGF2alpha, b.i.d., X 4 days had increased LH values on day 8, thus the PG appeared luteotrophic. Rats treated with 1 mg of PGF2alpha, b.i.d., X 4 days had decreased LH values on day 7 and the CL showed subsequent luteolysis. FSH levels were relatively constant during pseudopregnancy. However, all doses of PGF2alpha reduced FSH levels on day 7. An associated decrease in uterine weight occurred, possibly due to reduced follicular development. Prolactin levels fell in response to PGF2alpha treatment which undoubtedly contributed to the observed luteolysis. The signs of early cessation of pseudopregnancy were: increased serum FSH on days 9 and 10; increasing uterine weight; and the reappearance of follicular growth. These data suggest that PGF2alpha reduced the duration of pseudopregnancy primarily by inhibiting the secretion of FSH, LH and prolactin.