Abstract: The activation of cyclin dependent kinase 5 (Cdk5) depends on the binding of its neuronal specific activator Nck5a. The minimal activation domain of Nck5a has been experimentally determine comprise acid residues 150 to 291 of the protein. It has been demonstrated that a 28 residue peptide encompassing amino acid residues Ala 146 to Asp 173 of Nck5a is capable of binding Cdk5 and hence resulting in the inhibition of its Kinase activity. Additionally, this peptide could also inhibit Cdk2 with a similar potency as it does to Cdk5. The direct competition experiments showed that the Nck5a inhibitory peptide does not compete with Nck5a for Cdk5 or cyclin A for Cdk2. Steady state kinetic analysis indicated that the Nck5a peptide acts as a non-competitive inhibitor of Cdk5/Nck5a complex with respect to its substrate.