Previous investigators considered pig corpora lutea refractory to the luteolytic effects of prostaglandin (PG) F2 alpha before Day 12 of the estrous cycle. This study was designed to determine whether multiple injections of PGF2 alpha would result in a sustained reduction of serum progesterone and luteolysis, leading to significant shortening of the estrous cycle and interestrous interval. On Days 5-10 of an estrous cycle, gilts (n = 4) received injections of 12.5 mg PGF2 alpha (dinoprost tromethamine) i.m. every 12 h, or vehicle (PBS; n = 4) according to the same schedule. Mean interestrous interval in PGF2 alpha-treated gilts was reduced (p < 0.001) to 13.3 +/- 0.5 days compared with 19.8 +/- 0.6 days for control gilts. Serum progesterone declined below 1 ng/ml by Day 10.5 in PGF2 alpha-treated gilts compared to Day 17.5 in control animals. Serum concentrations of estradiol-17 beta (E2) reached maximal levels in PGF2 alpha-treated gilts earlier (Day 12.5) in the cycle than in control gilts (Day 19.5). Peak E2 and LH concentrations coincided with the periestrous period, suggesting that PGF2 alpha-induced estrus is accompanied by normal follicular development and ovulation. These results demonstrate that the pig is susceptible to the luteolytic effects of PGF2 alpha before Day 12 if repeated injections are given from Day 5 through Day 10.