Abstract Previous studies using the fresh water turtle Chrysemys picta have demonstrated the differential expression of the two progesterone receptor isoforms (PRA and PRB) in the liver during the turtle seasonal cycle, correlating with ER mRNA levels and hepatic vitellogenesis. During the inter-vitellogenic periods the ratio of PRB:PRA favors PRA, suggesting that the PRB:PRA ratio may be important in the regulation of vitellogenesis. Based on these and other studies we hypothesize that progesterone may have differential effects on the estrogen (E)-induced vitellogenin response (inhibitory or stimulatory), depending on the PRB:PRA ratio. In this study, we determined the expression pattern and the hormonal regulation of progesterone receptor (PR) isoforms in the liver, during the estrogen-induced vitellogenin response in female C. picta. Northern blot analysis was used to evaluate the changes in vitellogenin mRNA, estrogen receptor (ER) mRNA, and PR mRNA expression; Western blot to determine changes in PR isoform expression and a homologous ELISA for measurement of plasma vitellogenin. The expression of PR isoforms in the liver of female turtles at the mRNA and protein levels was differentially regulated by estradiol and progesterone. Estradiol treatment enhanced the transcription of PR mRNA isoforms and possibly translation and/or increased stability of PRB protein in the female turtle liver. In contrast, this hormone decreased PRA protein levels. Progesterone alone down-regulated progesterone receptor isoform A (PRA) and progesterone receptor isoform B (PRB) proteins equally, but did not affect PR mRNA levels. Estradiol markedly increased hepatic ER mRNA, vtg mRNA, and vtg in plasma, but this effect was not impacted significantly by progesterone. The results presented here indicate that in female turtles hepatic PRB:PRA ratios are markedly changed by estradiol treatment due primarily to a decrease in PRA. The change in the PRB:PRA ratio after hormonal treatment confirms that progesterone and estrogen exposure will be a determinant in the regulation of vitellogenesis, and, in turn, that the regulation of vitellogenesis will be determined by the ratio of PR isoforms and the physiological levels of steroid hormones.