Abstract: AIM: The interaction between phenothiazine drugs and DNA was studied with ethidium bromide(EB) probe by electronic absorption and fluorescence spectra. METHODS AND RESULTS: Both chloropromazine hydrochloride(CPZ) and trifluoperazine hydrochloride(TFP) in buffer solution at pH 7.4 can intercalate into DNA. Promethazine hydrochloride(PMZ), which has the similar group as CPZ, can weakly bound to DNA by mixture modes. CONCLUSION: This interaction can be inhibited if high concentration of ions exists.