Explants of human placental tissue harvested immediately after expulsion were used to determine differences between accumulation of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and polychlorinated dibenzo-p-dioxin (PCDD)-polychlorinated dibenzo-p-furans (PCDF) environmental mixture, and their influence on placental steroidogenesis. Explants were cultured in vitro for 5 days in media supplemented each day with either TCDD or a mixture of PCDD-PCDF. Media were collected every day for steroid content analysis by radioimmunoassay. At 24 h after the last treatment, the tissue was frozen for further analysis of the content of TCDD or other congeners present in the mixture. Determinations of TCDD and all 17 PCDDs and PCDFs were performed using gas chromatography equipped with DB-5 MS and DB-17 capillary columns. In the control tissue, the amounts of both TCDD and mixture components were close to the limit of detection of the method. In the treated tissue, the TCDD accumulation was 94% of the total exposure to TCDD. The most toxic congeners 2,3,7,8-TCDD, 2,3,7,8-tetrachlorodibenzofuran, 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD), 1,2,3,7,8-pentachlorodibenzo-p-furans (PeCDF) and 2,3,4,7,8-PeCDF showed the highest accumulation, which covered >50% of the total toxic equivalents present in this mixture. During the first 3 days of exposure to TCDD there was no effect on the conversion of dehydroepiandrosterone to oestradiol, whereas on days 4 and 5 of exposure, a twofold decrease in oestradiol secretion was observed. However, a small but significant increase in oestradiol secretion was noted at all times of exposure to the PCDD-PCDF mixture. All observed changes in oestradiol secretion were not accompanied by changes in progesterone secretion after exposure to TCDD or the PCDD-PCDF mixture. In conclusion, a high accumulation of TCDD in the placental tissue resulted in a decrease in oestradiol secretion and in vivo this could result in a decrease in blood flow through the placenta. From the mixture, PeCDD and PeCDF in the higher amount accumulated in the placental tissue caused an increase in oestrogen secretion and as a consequence could activate oxytocin secretion from the pituitary and early pregnancy outcome.