In the endometrium two enzymes are known to convert estradiol to its inactive metabolite estrone: microsomal 17 -hydroxysteroid dehydrogenase type 2 (17 -HSD2) and peroxisomal 17 -HSD4. In order to elucidate the particular function of each of these two different enzymes, the human endometrial epithelial cell lines HEC-1-A and RL95-2 were examined with respect to the expression of 17 -HSD isozymes. They were compared with human endometrium in vivo. Non-radioactive in situ hybridization revealed both enzymes in glandular epithelial cells of human endometrium. The two cell lines were screened for mRNA expression of 17 -HSD 1-4 by RT-PCR and Northern blot. 17 -HSD2 and 4 could be detected by either method, 17 -HSD1 only by RT-PCR, 17 -HSD3 not at all. Both cell lines were proven to have no receptor for progesterone which is known as a physiological inducer of several 17 -HSD isozymes. To study the regulation of 17 -HSD2 and 17 -HSD4, the concentration of fetal calf serum in the cell culture media was reduced stepwise to 0·3% by dilution with a defined serum replacement. This treatment led to an inhibition of 17 -HSD2 mRNA expression and an increase in the mRNA expression of 17 -HSD4. Concomitantly, distinct morphological changes were observed, such as a decrease in the number and length of microvilli and a decrease in the formation of domes on top of the monolayers.The endometrial epithelial cell lines HEC-1-A and RL95-2 represent a suitable in vitro model for further studies of the differential expression of the major endometrial HSD isozymes, independent of the effect of progesterone.