Alterations of H-and M-isozymes of Lactic Dehydrogenase(LDH) were observed in the various tissues after exposing the rats to 50ppm and 250ppm of sulfur dioxide. These isozymes of the respective tissue were separated by Diethlaminoethyl (DTAE)-cellulose from the tissue homogenates of brain, lung and muscle, presenting the activities by rate of reduction of nicotinamide-adenine-dinucleotide ($NAD^+$). Pure LDH and the coenzyme ($NAD^+$) were directly treated with sulfur dioxide in vitro in order to find out the direct to sulfur dioxide on LDH and $NAD^+$ and the results were as follows. 1. In the normal tissues, the H-isozyme activity was dominant in the brain and heart, and the M-isozyme in the muscle. 2. In the lung tissue of normal rats, there was no difference between the activity of H-and M-type of LDH. 3. When rats inhale sulfur dioxide gas in concentration of 50ppm and 250ppm, it appeared that the H-type tend to be suppressed in aerobic tissues and the M-type in anaerobic tissues. 4. In the lung tissue exposed to sulfur dioxide, both the LDH activities were suppressed. 5. It seems that LDH and the coenzyme ($NAD^+$) are not directly affected by exposing in sulfur dioxide gas.