Progesterone (P) is known to regulate sex steroid receptors in uterine cells. However, its precise regulation at the messenger ribonucleic acid (mRNA) level is unclear. In this study we examined the effects of P and testosterone (T) on the regulation of sex steroid receptors in cultured human endometrial stromal cells (ESC), using the quantitative reverse transcriptase polymerase chain reaction method. We isolated ESC from human endometrial tissues and cultured them with or without P (10(-6) mol/L) or T (10(-8) mol/L) for 9 days. Incubation with P decreased progesterone receptor (PR), estrogen receptor, and androgen receptor mRNA levels in cultured human ESC to 0.56 +/- 0.04-, 0.53 +/- 0.08-, and 0.84 +/- 0.04-fold (mean +/- SE), respectively. T also decreased PR, estrogen receptor, and androgen receptor mRNA levels in cultured human ESC to 0.48 +/- 0.06-, 0.52 +/- 0.05-, and 0.82 +/- 0.04-fold (mean +/- SE), respectively. These decreases by P and T occurred in a dose-dependent manner. We also examined the sex steroid receptor levels in human ESC cultured for 0, 3, 6, and 9 days. The PR mRNA level in ESC without P was increased in a time-dependent manner. This increase was also inhibited by P, and the mRNA level in the presence of P was almost constant throughout the culture period. Our results demonstrated that P or T is a regulator of sex steroid receptors in ESC and that this regulation may influence the responsiveness to P of decidual change in ESC.