A rapid and sensitive high performance liquid chromatographic (HPLC) method is described for the quantitation of cinnamaldehyde (CNMA) in rat blood at concentrations of 0.1-100 micrograms/mL. One of the metabolites of CNMA, cinnamic acid, can also be quantified simultaneously. CNMA is unstable in rat blood, probably because of rapid oxidation to cinnamic acid by enzymatic catalysis and nonenzymatic Schiff base formation with free amine groups of blood proteins. The disappearance of CNMA from rat blood follows first-order reaction kinetics with a half-life of 9 min at room temperature. The current analysis method involves the addition of an agent that will prevent CNMA degradation by denaturing protein and competitively blocking nucleophilic addition reactions, resulting in the nearly complete recovery of CNMA from blood. Recovery of cinnamic acid was approximately 80% at concentrations of 1-10 micrograms/mL.