To determine the transfer and metabolism of TRH by human fetal membranes, the bidirectional transport and uptake of TRH was investigated by adding 125I-labeled TRH (100,000 cpm) or commercial TRH either to the maternal or the fetal compartment of an in vitro model of cultured human fetal membranes obtained from term and preterm placenta. Transmembrane transfer was also studied in the presence of 200 microM p-hydroxy-mercuriphenyl-sulphonic acid (p-HMSA), a dipeptidase enzyme inhibitor. Creatinine and heparin were used as an internal markers. Metabolites of TRH were separated from intact molecules by gel filtration on Sephadex G-10. The structural integrity of the membrane was confirmed by electron microscopy. The transmembrane transfer of radiolabeled and commercial TRH were comparable across both preterm and term placenta. When transport was studied from the maternal to fetal side, the maternal concentration of TRH declined rapidly from 100% at time 0 to 19.31 +/- 2.26% at 8 h with a concomitant increase in the fetal concentration from undetectable to a maximum of 2.56 +/- 0.38% with a fetomaternal ratio of 0.16 +/- 0.01. Transfer of TRH from the fetal to maternal compartment was similar to that of maternal to fetal. Chromatography of maternal and fetal media showed that TRH was metabolized by the membrane into small molecular weight fragments. Treatment of the membrane with p-HMSA increased TRH transport from the maternal to fetal compartment to 18.12 +/- 0.91 (P < 0.001) with an fetomaternal ratio of 0.35 +/- 0.02 (P < 0.001). Although transmembrane transfer of TRH from the fetal to maternal side was also increased by p-HMSA, levels achieved were less than that from maternal to fetal (12.26 +/- 1.50%; P < 0.05). These results suggest that the human fetal membrane acts as an enzymatic barrier to the bidirectional transfer of TRH from 24 weeks gestation.