Corticosteroid-binding globulin (CBG or transcortin) is a specific plasma glycoprotein, which binds steroid hormones (cortisol, corticosterone, and progesterone), and plays a role in transporting these steroids, altering their concentrations in blood, and influencing their biological actions. CBG has been previously shown to be synthesized in the liver, but recently it has been reported that immunoreactive CBG is localized in target tissues. In the present work, CBG mRNA was detected in normal human endometrial tissues by Northern blot analysis and reverse transcription-polymerase chain reaction. Its level was higher (P < 0.05) in the secretory phase than in the proliferative phase. In the secretory phase, the endometrial CBG mRNA level was negatively correlated with the serum progesterone level (P < 0.01). While there was no positive correlation between the levels of endometrial CBG mRNA and serum estradiol (E2), there was a positive correlation between the endometrial CBG mRNA level and the serum E2/progesterone ratio (P < 0.05). These findings suggest that CBG is synthesized in the uterine endometrium, predominantly in the secretory phase, and that the serum E2/progesterone ratio exerts an influence on the synthesis of intracellular CBG.