The 125I-labeled endothelin-1 ([125I]ET-1) binding sites in microvillous membranes from early gestation and term human placentas were investigated. The Kds for [125I]ET-1 binding to early gestation (68 +/- 15 pmol/l) and term (45 +/- 8 pmol/l) microvilli (n = 4) were not significantly different. The density of binding sites decreased significantly, from 243 +/- 80 fmol/mg protein in early gestation microvilli to 54 +/- 10 fmol/mg protein in term microvilli. The endothelin (ET) receptor (ET-R) subtype profiles were determined by competition binding studies with unlabeled ET-1, ET-3, and selective agonists and antagonists for ETA-R and ETB-R. In early gestation placental microvilli, we observed the presence of 72%) ETB-R, (mainly ETB2-R subtype), and 28%. ETA-R. Only ETB-R (mainly the ETB2-R subtype) was present in term placental microvilli. We suggest that the ETB-R on the placental microvillous membrane is involved in specific trophoblastic functions and may play a major role in ET clearance by modulating the amounts of ETs in the maternal intervillous blood space.