Peptide derivatives serving as thrombin receptor antagonists are disclosed, which bear specificity for the cellular thrombin receptor without interfering with the catalytic activities of thrombin. The derivatives generally have formula (I) in which R?1 and R3¿ are amide linkages, N-alkylamide linkages, or isosteric replacements of such linkages; R2 is either a neutral amino acid side chain or a hydrophobic radical; R4 is hydrophobic radical; R5 is CO, CH¿2? or SO; X is either of formula (II) in which R?6 and R7¿ are H, alkyl, cycloalkylalkyl, alkoxyalkyl, alkylthioalkyl or arylalkyl, and R8 is a hydrophobic radical, or a hydrophobic residue; Y is alkoxy, hydroxy, amino, alkylamino, dialkylamino, in which any of the alkyl groups may be substituted with a basic moiety; Z is an amino acid or peptide residue; and m is zero or one.