The distribution of the endothelin (ET)-receptor subtypes ET(A) and ET(B) in the rat testis and their involvement in the secretory response of Leydig cells to ET-1 have been investigated by the use of specific ligands. Autoradiography showed that [125I]ET-1 binding was intense in the interstitial area of the testis, containing Leydig cells, and virtually absent in the walls of seminiferous tubules. Labelling was almost completely displaced by BQ-123, a selective ETA receptor antagonist, while sarafotoxin-6C and BQ-788, two specific ET(B) ligands, were ineffective. ET-1 concentration-dependently enhanced testosterone secretion of dispersed rat Leydig cells, and the response was suppressed by BQ-123, but not by BQ-788. Both antagonists per se did not affect either basal and hCG stimulated secretion of Leydig cells. Taken together our findings indicate that rat Leydig cells are mainly, provided with ETA, and that this ET-receptor subtype mediates their secretory response to ET-1.