Pharmacological profiles of endothelin (ET) receptors in the isolated rabbit iris sphincter were characterized. ET isopeptides caused dose-dependent contraction of the preparation. The respective EC50values for ET-1, ET-2 and ET-3 were 39.4, 58.0 and 84.3 nM, so that ET-1 was twice as potent as ET-3. Sarafotoxin (SRTX) -b, an ETA/ETBnon-selective agonist, caused very potent contraction with an EC50of 1.13 nM. However, selective ETBreceptor agonists SRTX-cand IRL 1620 showed no contractile activity up to 1μM.BQ-123, a selective ETAreceptor antagonist, shifted the dose-response curves of ET isopeptides to the right. The pA2value for ET-I was 5.52 with a slope of 1.06, which is not different from unity, and the pKBvalue for ET-2 was 5.06. Interestingly, very low doses of BQ-123 antagonized responses to ET-3 and SRTX-b, with a Schild plot slope of approximately 0.7 which is significantly different from unity, suggesting receptor heterogeneity. The abscissal intercepts of the Schild plots were–9.29 for ET-3 and–8.53 for SRTX-b. FR 139317, another ETAreceptor antagonist, also preferentially antagonized responses to ET-3. RES 701-1, a selective ETBreceptor antagonist, did not shift dose-response curves for ET-1 and ET-3.These results suggest that ET receptors in the rabbit iris sphincter muscle cannot be classified into the ETA, ETBor ETcreceptor subtypes, so far established. When compared to the established receptor subtypes, ET receptors in this preparation were quite different from the ETBreceptor, but apparently showed a pharmacological profile most similar to the ETAreceptor, suggesting the presence of heterogeneous and atypical ETAreceptors.