While it is well known that progestins have significant effects on estrogen-dependent processes in the uterus, very little is known about the molecular details of these effects. To understand these processes we have developed an immunocytochemical staining technique to help clarify the progestin regulation of estrogen receptor (ER), and we have also studied the effects of progestins on estrogen-dependent specific protein synthesis and secretion in the immature rat uterus. The immunocytochemical method for identifying estrogen receptor in tissue sections uses biotinylated monoclonal antibody to the estrogen receptor protein and shows a largely nuclear localization of the receptor protein in the rat uterine endometrium and myometrium. This method can be used to explore changes in the intercellular and intracellular localization of the uterine ER during progestin action. In an effort to correlate these changes with specific biological responses in the uterus we have been studying the effects of the administration of estrogens and progestins in vivo on the synthesis of specific proteins in uterine target cells in vitro. We previously reported that one-dimensional SDS-PAGE analysis of labeled secreted uterine proteins and cellular proteins extracted from the luminal epithelium and from the stroma plus myometrial fractions of the uterus revealed that estradiol-17 beta preferentially stimulated the synthesis of 110,000, 74,000, and 66,000 dalton secreted proteins, and 180,000 and 110,000 dalton epithelial proteins. We found that while progestins administered alone in vivo did not have any stimulatory effect on the synthesis of these secreted or cellular uterine proteins, concomitant administration of either progesterone or megestrol acetate and estradiol in vivo substantially reduced the estrogen-stimulated increase in labeling of the 110,000, 74,000, and 66,000 dalton secreted uterine proteins in vitro. In animals first primed with the progestin prior to combined estrogen/progestin treatment, the progestins were even more effective antagonists of estrogen. In addition, progestins alone given 24 hours after estradiol substantially inhibited the estrogen-stimulated synthesis of these proteins. These results suggest that progestins can both modulate the initial level of estrogen stimulation of synthesis of specific proteins and selectively terminate their synthesis, by affecting the continued transcription of their estrogen-regulated genes, altering the processing, half-life, or translation of their mRNAs, or by modulating the translation or the posttranslational processing of the nascent peptide chains.