We characterized 125I-ET-1 binding to renal microvascular membranes isolated from the rat, a species showing ETB receptor-mediated renal vasoconstriction, and the rabbit in which ET-induced renal vasoconstriction is mediated by the ETA receptor. In both species, 125I-ET-1 bound in a manner consistent with a single high-affinity site. Scatchard analysis yielded Kd and Bmax values of 20.1 +/- 0.4 pM and 1343 +/- 64 fmol/mg for the rat and 21.5 +/- 0.9 pM and 810 +/- 64 fmol/mg for the rabbit. Competition binding studies with several selective (sarafotoxin 6c, BQ123) and mixed ETA/ETB (SB 209670) ET receptor ligands showed that the renal microvasculature from both species contain ETA and ETB receptors in a proportion of 40:60. In the rat, the proportion of ETA receptors was higher in the microvasculature than in glomeruli and inner medullary collecting duct cells both of which contained > 80% ETB receptors. In the rabbit, the proportion of ETA/ETB receptors was similar in the microvasculature and inner medullary collecting duct cells (approximately 40:60), whereas glomeruli contained 80% ETB receptors. Although ET-induced renal vasoconstriction in the rat and rabbit is mediated by different ET receptor subtypes, the proportion of ETA to ETB receptors is the same in the renal microvasculature from these species.