1. This study was performed to clarify whether the endothelin (ET) receptor subtypes mediating two pharmacologically heterogeneous response to ETH receptor agonists in normal mice are the product(s) of a single ETB receptor gene. 2. Vasodilator responses to sarafotoxin S6c (S6c) in the thoracic aorta and contractile responses to ET-1 and IRL1620 in the stomach were examined in tissues from normal and ETB receptor gene knockout mice, in the absence and presence of an ETA receptor antagonist, BQ-123, or an ETA/ETB receptor antagonist, PD142893. 3. In the normal mouse aorta precontracted with phenylephrine, S6c (0.1-100 nM) caused concentration-dependent relaxations (pD2 = 8.4). BQ-123 had no effect on these responses. However, PD142893 almost abolished the relaxations induced by 0.1-300 nM S6c. 4. In aortae taken from ETB receptor gene knockout mice, S6c up to 1 microM failed to cause relaxations, confirming that ETB receptors are involved in mediating this response. 5. In normal mouse gastric fundus, 0.1 nM-1 microM ET-1, S6c or IRL1620 caused dose-dependent, BQ-123-insensitive contractions, which were much more resistant to PD142893 than S6c-induced relaxations of the aorta. The pD2 values for S6c in the absence and presence of PD142893 (10 microM) were 8.12 +/- 0.11 and 7.70 +/- 0.11, respectively. 6. In the gastric fundus of the ETB receptor gene knockout mouse, S6c and IRL1620 caused no contractions. ET-1 (0.1 nM-1 microM) caused contractions sensitive to both BQ-123 and PD142893, indicating that only ETA receptors mediate ET-1-induced contractions of the knockout mouse gastric fundus. 7. Since both the PD142893-sensitive vasodilator response of the aorta and the PD142893-resistant contractile response of the gastric fundus to S6c were completely absent in the ETB receptor gene knockout mouse, we conclude that the two pharmacologically heterogeneous responses to S6c are mediated by receptors derived from the same ETB receptor gene.