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Receptors involved in endothelin induced relaxation were characterized on isolated circular segments of guinea-pig trachea. The motor responses to endothelin-1 (ET-1), ET-2 and ET-3 as well as the effects of the ETB-receptor agonist, BQ 3020, and the ETA-receptor antagonist, FR 139317, were tested. The responses obtained were analysed in relation to the nitric oxide (NO) synthase inhibition by NG-monomethyl L-arginine (L-NMMA). In submaximally precontracted tracheal segments ET-1 and ET-2 induced concentration-dependent dilatations. ET-3 induced a biphasic response in precontracted segments, at low concentrations a small contraction followed by a dilatation at higher concentrations. FR 139317 blocked the low concentration induced dilatation by ET-1/ET-2, unmasking a contractile response. The dilatation induced by higher ET-1/ET-2 concentrations was unaffected by FR 139317 which also did not affect the dilatory response of ET-3. These results indicate the presence of two dilatory receptors. One of these was of ETA type, since it was antagonized by FR 139317 while the other receptor probably was of the ETB type, since all three endothelins in the moderate concentration range caused a uniform dilatation which was unaffected by FR 139317. Furthermore, BQ 3020 induced potent relaxation of the precontracted segments. In resting tracheal segments all three endothelins caused an identical contraction, suggesting a contractile ETB-receptor. However, BQ 3020 along with another ETB-agonist, IRL 1620, failed to induce contraction which indicates the possibility of two different subtypes of the ETB-receptor, one involved in dilatation and the other in contraction. Experiments with the nitric oxide synthetase inhibitor L-NMMA on precontracted segments showed a similar change as when FR 139317 was used. This suggests that the dilatory ETA-receptor depends on nitric oxide (NO) for mediating the dilation. On the other hand, the dilatory ETB-receptor response was unaffected by L-NMMA which suggests a mechanism unrelated to NO.