Spectroscopic (fluorescence and absorptiometric) techniques were used to study the interaction between peroxynitrite, a known biological stressor, and curcumin, a potent biological antioxidant. In both techniques, the observed signals increased asymptotically until the concentration of peroxynitrite equals that of the curcumin, which was held constant at 1 x 10(-5) M. However, there was a shift in fluorescence wavelength after initial apparent oxidation of the hydroxyl group. This is perhaps due to nitration of the phenoxyl group of curcumin. The obtained data, using these techniques, suggest a second order reaction rate for the nitration of the curcumin by peroxynitrite. The biomolecular interaction or association constant obtained was 1.2 x 10(6)/M-s using the fluorescence technique and it was 3.6 x 10(6)/M-s using the absorptiometric technique. These values are in good agreement with literature values.