A bioassay for progesterone activity in dogs was established based on uterine weight (mg/kg body weight) in immature beagles administered progesterone for 10 days starting 9 days after priming with estradiol cypionate (50 micrograms/kg, im). Progesterone doses of 0, 0.17, 0.5, 1.5, 13.5 and 40.5 mg/kg per day, im, produced dose-dependent increases in the weights of uterine horns obtained after 5 or 10 days of treatment. The total uterine responses (horn removed at 5 days plus horn and fundus removed at 10 days) to those were (mean +/- SEM) 374 +/- 33, 465 +/- 97, 684 +/- 68, 795 +/- 96, 1005 +/- 38, 1232 +/- 15 mg/kg, respectively. Responses to the 13.5 mg/kg per day dose of progesterone in dogs given the steroid antagonist RU486 at daily oral doses of 5, 20 and 50 mg/kg were reduced to values of 634 +/- 24, 464 +/- 74 and 468 +/- 18 mg/kg, respectively, vs 1005 +/- 38 mg/kg in controls. Mean progesterone levels were 27 +/- 1 micrograms/l. The RU486 did not produce any consistent alterations in serum cortisol levels. The results suggest that, in immature bitches, uterine weight changes can be used to bioassay progestin activity following estrogen priming, RU486 is more potent as an antiprogestin than as an antiglucocorticoid, and RU486 at oral doses of 5 and 20 mg/kg exerts submaximal and maximal antiprogestin activity, respectively, in the presence of physiological levels of progesterone.