Tetracyclinea ntibiotics could be degraded after being heated in boiling-water bath in alkaline solution (in KOH medium). The fluorescence excitation and emission wavelengths of the degradation products changed and the fluorescence intensity increased by 5-190 folds as compared with that of antibiotics themselves. It was considered that there occurred a nucleophilic ring rupture reaction to form the degradation products with lactonic structure. The interaction of the lone pair electrons of oxygen on the degradation products with the conjugated n electrons induced the increase of the delocalization for the conjugated electron system in the molecule and thus enhanced the fluorescence intensity.