Novel peptide inhibitors of GSK-3, compositions containing same and uses thereof are disclosed. The novel peptide inhibitors are substrate-competitive inhibitors and have an amino acid sequence designed so as to bind to a defined binding site subunit in GSK-3. Also disclosed are GSK-3 substrate competitive inhibitors which bind to the defined binding site subunit in the enzyme. Also disclosed are mutants of GSK-3 and uses thereof for identifying a putative GSK-3 substrate competitive inhibitor.