A "smart" PARACEST MRI contrast agent was synthesized to detect caspase-3, which is an important biomarker in apoptosis. The well-known caspase-3 substrate, DEVD (Asp-Glu-Val-Asp), was elongated using the amino group on one sidearm of lanthanide ligand anchored on the polymer support. The amide of DEVD-(Tm-DOTA) showed a PARACEST effect with MR saturation at -51 ppm. DEVD-(Tm-DOTA) amide was successfully cleaved by caspase-3, exposing the free amine group, which showed PARACEST with saturation at +8 ppm. The enzymatic activity of caspase-3 can be detected by the change in PARACEST effect caused by this biotransformation.