To investigate the mechanism of induction of uteroglobin by progesterone, the relationship between uteroglobin mRNA (mRNAug) activity and nuclear progesterone receptor levels has been examined in rabbit endometrium during early pregnancy. mRNAug activity was assessed by translation in vitro of poly A-rich endometrial RNA and immunoprecipitation of the synthesized peptides using anti-uteroglobin antibodies. Progesterone receptor was determined in purified endometrial nuclei by Scatchard analysis of the specific binding of 3H-R5020 under exchange conditions. mRNAug activity reached a peak on Day 4 of pregnancy and declined thereafter up to Day 8. Nuclear progesterone receptor levels showed a slight rise on Day 2, then returned to baseline levels. When mRNAug activity was high, progesterone receptor levels had declined. A cause-and-effect relationship between nuclear progesterone receptors and specific mRNA or protein synthesis remains to be established.