Two analogues of dioxobenzothiazole,2-chloro-5-dodecylmercapto-6-methyl-4,7-dioxobenzothiazole (2-C1-DMMDBT)and 2-chloro-5-butylamino-6-methyl-4,7-dioxobenzothiazole(2-Cl-BAMDBT), were synthesized.Their inhibitory properties on the enzymes of mitochondrial respiratory chain were studied in heart muscle preperation.Both of them show the inhibitory effect on the succinate oxidase, ubiquinol oxidase,NADH oxidase but not the cytochrome c oxidase.These results indicate that the inhibitory sites of both compounds are located on the area of ubiquinone reactions. The substitution of -SR and-NHR at 5-position of the benzothiazole ring makes the two compounds of 2-CI-DMMDBT and 2-Cl-BAMDBT show different behavior on the inhibition of NADH-Q reductase. The stronger inhibitory effect of 2-Cl-DMMDBT than 2-Cl-BAMDBT is probably due to the longer hydrophobic side chain of 2-Cl-DMMDBT，because the reactive sites of ubiquinone are all in the membrane of the mitochondria.