T he effects of estrogen and progesterone on uterine alpha-adrenergic receptors were investigated by direct receptor-binding studies. Immature female rabbits were primed with estrogen by intramuscular injections for 4 days. Other rabbits were primed with progesterone by injections of estrogen for 4 days followed by injections of progesterone for 4 days. The alpha adrenergic antagonist, [3H]dihydroergocryptine, was used to directly assess the number and affinity of alpha adrenergic receptors in membranes derived from estrogen-and progesterone-primed uteri. Membranes from estrogen-primed uteri contained 257 +/- 52 fmol of [3H]dihydroergocryptine-binding sites per mg protein whereas membranes from progesterone-primed uteri contained 83 +/- 11 fmol of of binding sites per mg protein. This reduction of alpha adrenergic receptor-binding sites by progesterone was statistically significant (P less than 0.02). In contrast, no significant difference in the binding site affinity was observed between the estrogen- and progesterone-primed groups. The progesterone-induced decrease in the number of uterine alpha adrenergic receptors provides a potential explanation for the reduced alpha adrenergic contractile response to epinephrine in the progesterone-primed myometrium.