Experiments were conducted to determine if prostaglandin F2alpha (PGF2alpha) is luteolytic in swine. In Experiment 1, four bilaterally hysterectomized gilts were injected with PGF2alpha at 0800 (10mg) and 2000 hours (10mg) and four gilts received .9% saline at the same times on day 17 after onset of estrus. Treatments were reversed in the two groups of gilts 21 days later. All eight PGF2alpha treated gilts exhibited estrus an average of 88.0 +/- 13.5 hours after treatment and average duration of estrus was 66.0 +/- 16.4 hours. Saline treated controls did not exhibit estrus. Two additional gilts were hysterectomized bilaterally and the saphenous artery catheterized on day 7 after onset on estrus. PGF2alpha injected on day 17 resulted in a precipitous decline in plasma progestin concentration and onset of estrus by 110 and 90 hours in gilts 1 and 2, respectively. Another bilaterally hysterectomized gilt, with CL marked with India ink, received PGF2alpha on day 17. Estrus occurred 92 hours later and, on day 4, regression of marked CL to corpora albicantia and presence of newly formed CL was confirmed at laparotomy. In Experiment 2, 12 bilaterally hysterectomized gilts were treated with PGF2alpha at 0800 (10mg) and 2000 hours (10mg) on either day 8, 11, 14 or 17 after onset of estrus. None of the gilts treated on days 8 and 11 exhibited estrus. Two of three gilts treated on day 14 and all three gilts treated on day 17 exhibited estrus at an average of 116.0 +/- 9.8 hours post-treatment. Average duration of estrus was 49.6 +/- 8.8 hours.