The activity of natural endothelins such as ET-1, ET-2, ET-3 and of sarafotoxin S6b (SRFTX) as compared to that of the C-terminal hexapeptide ET-(16-21) was investigated in several smooth muscle preparations in the presence of indomethacin, captopril, bestatin and thiorphan. In some tissues (rat thoracic aorta, guinea-pig ileum, human urinary bladder, renal pelvis or renal artery), ET-(16-21) had little if any agonist activity. In other preparations (guinea-pig bronchus, rat vas deferens, rabbit pulmonary artery) ET-(16-21) was a full agonist. The amidated form of ET-(16-21) was either inactive or less potent than ET-(16-21). These findings provide evidence that at least two receptors exist for peptides of the ET family; these receptors were termed ETA and ETB. ET-(16-21) is a full agonist at ETB receptors while being inactive or weakly active at ETA receptors. The free acid of tryptophan in position 21 plays a key role in the activity of these peptides at ETB receptors.