Peptides corresponding to the C-terminal 16-21 hexapeptide of the endothelins (-His-Leu-Asp-Ile-Ile-Trp) and sarafotoxins (a-c) (-His-Gln-Asp-Val-Ile-Trp) were prepared to study the role of the individual amino acids in receptor recognition and activation. Receptor binding in rabbit aorta, rabbit pulmonary artery, and rat heart ventricle is reported for all analogues. In addition, selected C-terminal hexapeptides have been evaluated functionally in two tissues (rabbit pulmonary artery and rat left atria). The C-terminal carboxylate, indole nitrogen, and nature of the aromatic residue are all important for receptor binding, but N-terminal acetylation has no effect. L-Amino acids are required in positions 19 and 21, whereas D-amino acids are tolerated in 17 and 18. D-Amino acids in positions 16 and 20 enhance the binding affinity of the hexapeptide in all three tissues. The nature of the basic residue at position 16 is important. Glu and Asn are acceptable substitutions for Asp18, although Ala leads to a substantial loss in binding. The binding of the C-terminal hexapeptide of SRTX-a, -b, and -c is less than ET[16-21] and this appears to be primarily due to the substitution of Gln for Leu17. None of the 16-21 hexapeptides showed any functional activity in the tissues studied.