CDK5/p35 is a cyclin-dependent kinase essential for normal neuron function. Proteolysis of the p35 subunit in vivo results in CDK5/p25 that causes neurotoxicity associated with a number of neurodegenerative diseases. Whereas the mechanism by which conversion of p35 to p25 leads to toxicity is unknown, there is common belief that CDK5/p25 is catalytically hyperactive compared to CDK5/p35. Here, we have compared the steady-state kinetic parameters of CDK5/p35 and CDK5/p25 towards both histone H1, the best known substrate for both enzymes, and the microtubule-associated protein, tau, a physiological substrate whose in vivo phosphorylation is relevant to Alzheimer's disease. We show that the kinetics of both enzymes are the same towards either substrate in vitro. Furthermore, both enzymes display virtually identical kinetics towards individual phosphorylation sites in tau monitored by NMR. We conclude that conversion of p35 to p25 does not alter the catalytic efficiency of the CDK5 catalytic subunit by using histone H1 or tau as substrates, and that neurotoxicity associated with CDK5/p25 is unlikely attributable to CDK5 hyperactivation, as measured in vitro.