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1. 1. The in vitro effect of progesterone on norepinephrine (NE) release and contractile activity was analyzed in uterine horns from estrogen-primed and progesterone-primed mice. 2. 2. Progesterone (6–10 nmol/ml) evoked the release of [ 3 H]NE above basal levels from uterine horns in both experimental conditions, the effect of progesterone on estrogen-primed being more important than on progesterone-primed mice uterus. 3. 3. Progesterone also increased electrically evoked [ 3 H]NE release in estrogen-primed uterine tissue, nevertheless no effect was observed in progesterone-primed ones. 4. 4. Progesterone (0.6–10 nmol/ml) inhibited uterine horn isometric contractions only in estradiol-primed mice. This effect was partially blocked in uterine horns from reserpine-treated mice and when propanolol (1 μM) was added to the preparation of estradiol-primed mice uterus.