Pregnant rats were injected with mifepristone (RU 486) on Day 15 of pregnancy. The force and frequency of uterine contractions, recorded by a microballoon technique, were significantly greater at 12, 24 and 36 h in treated than in control rats (11.9 +/- 1.9 vs. 8.9 +/- 1.2 units, 17.7 +/- 3.0 vs. 10.5 +/- 2.3 units and 16.8 +/- 2.9 vs. 8.8 +/- 1.8 units for force and 51.3 +/- 9.1 vs. 29.4 +/- 3.8/h, 35.4 +/- 6.4 vs. 22.1 +/- 4.9/h and 35.6 +/- 3.2 vs. 24.6 +/- 4.6/h for frequency, respectively). There was no significant difference in concentrations of prostaglandin (PG) E-2 or PGF-2 alpha between treated and control rats at 12 h and 24 h after injection. At 36 h, 7 of 12 rats were aborting and uterine PG concentrations in these were significantly greater than in the others (1.5 +/- 0.2 vs. 0.9 +/- 0.2 ng PGE-2/g and 38.6 +/- 19.2 vs. 16.9 +/- 5.4 ng PGF-2 alpha/g), but there was no significant difference between control and treated rats that were not aborting. Concentrations of PGE-2 and PGF-2 alpha were significantly higher at 48 h when abortion had occurred in all animals (6.5 +/- 2.6 vs. 2.4 +/- 1.7 ng PGE-2/g and 30.4 +/- 8.9 vs. 9.3 +/- 5.6 ng PGF-2 alpha/g). Thus, the increase in uterine contractile activity induced by mifepristone preceded significant changes in concentrations of PGE-2 and PGF-2 alpha in the uterus and so could not have been caused by these changes.