One hundred Suc‐X‐Y‐Ala‐pNA peptides (SUC: succinyl, pNA:p‐nitroanilide, X, Y: Gly, Ala, Val, Leu, Ile, Phe, Pro, x‐aminobutyric acid, norvaline, norleucine) were synthesized and their reaction constants with porcine pancreatic elastase (Km,KcatandKcat/Km) were determined. These reaction constants were quantitatively analyzed using the Free–Wilson/Fujita–Ban method. The contribution of amino acid side chains to the reaction constantsKm,KcatandKcat/Km), expressed logarithmically, was found to be additive. On the other hand, 19 elastase inhibitors of the general formula CF3CO‐X‐Y‐Ala‐pNA (X,Y: ten amino acids) were synthesized, and their inhibition constants were compared with the Michaelis constant for the corresponding substrates and analyzed using free‐energy‐related substituent constants. In the analysis of amino acid side chains in the Y position, theKivalue of the inhibitor was generally correlated to theKmvalue of the substrate, which corresponded to the inhibitor, thus confirming the validity of the equationThis study may serve as a prototypical approach to unraveling structure–activity relationships of peptide substrates and inhibitors of medicin