The cytosolic peptidyl‐prolyl cis/trans isomerase cyclophilin from pig kidney can accelerate catalytically the cis/trans isomerization of prolyl peptide bonds. One‐ and two‐dimensional 1H NMR spectroscopy was used to prove that the polypeptide hormone calcitonin is a substrate for cyclophilin. Isomerization of only one of the two prolyl peptide bonds is catalyzed significantly. The efficiency of catalysis was calculated by lineshape analysis and NOESY spectroscopy. Cyclosporin A completely blocks the effect of the enzyme on the conformational dynamics of the polypeptide.