The output of prostaglandin (PG) F and PGE-like material into the solution bathing uterine horns isolated from adult ovariectomized and immature rats under the influence of progesterone, was studied. The injection of progesterone (0.5; 1.0; 2.0 or 4.0 mg) 6 hours prior to sacrifice enhanced significantly the release of PGF-like substance without modifying that of PGE. The augmentation of PGF was detected as early as one hour after injecting 4.0 mg of progesterone and remained elevated at 2,4,6, 12 and 24 hours, following the treatment. Puromycin (50 mg/Kg), injected 6 hours before killing, failed to alter the release of PGF-like substance but clearly blocked the stimulating effect of the hormone. In addition, progesterone also enhanced significantly the release of PGF-like material by horns isolated from immature animals. The results suggest that progesterone receptors do not appear to be important for the described effect of the hormone because the preparations employed in the present study have a very small content of these receptors. Alternatively, it can be hypothesized that only a reduced number of progesterone receptors are sufficient for the action of the hormone augmenting the output of PGF-like material from the uterus.