Administration of peptide YY(3-36) (PYY(3-36)) to fasting humans or mice shortly before re-feeding effectively reduced their food intake, but PYY(3-36) exhibited a functional half-life of only approximately 3 h. Attachment of poly(ethylene glycol) to proteins and peptides (PEGylation) prolongs their half-life in vivo, but completely inactivated PYY(3-36). We developed a reversibly PEGylated PYY(3-36) derivative by coupling it to a 40 kDa PEG through a spontaneously cleavable linker. The resulting conjugate (PEG(40)-FMS-PYY(3-36)) gradually released unmodified PYY(3-36) in vivo, exhibiting an eightfold increase in its functional half-life, to approximately 24h. This long-acting PYY(3-36) pro-drug may serve as an effective means for controlling food intake in humans.