Accurate, sensitive, and simple spectrophotometric and spectrofluorimetric methods were developed for the determination of gliclazide in pharmaceutical formulations and biological fluids. Both methods are based on a coupling reaction between gliclazide and 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole in borate buffer, pH 7.8, in which a yellow reaction product that can be measured spectrophotometrically at 400 nm was developed. The same product exhibited a yellow fluorescence at 470 nm upon excitation at 400 nm. The absorbance-concentration plot was rectilinear over the range of 2-20 microg/mL with minimum detectability [signal-to-noise (S/N) ratio = 2] of 0.2 microg/mL (6.18 x 10(-7) M); the fluorescence-concentration plot was rectilinear over the range of 0.2-2.5 microg/mL with minimum detectability (S/N = 2) of 0.02 microg/mL (6.18 x 10(-8) M). The different experimental parameters affecting the development and stability of the color were carefully studied and optimized. Both methods were successfully applied to the analysis of commercial tablets. The results were in good agreement with those obtained with the official and reference spectrophotometric methods. A proposal of the reaction pathway was presented.