o-Dichlorobenzene (o-DCB) was emulsified in a 2.5% gum arabic solution and given orally to rats for 60 and 120 days. The effects of o-DCB on rats were compared with control rats given water containing the same percent of gum arabic. The following results were obtained: 1. The livers of rats administered o-DCB were slightly heavier than those of control rats. 2. Triglyceride levels of rats administered o-DCB were higher than those of control rats. The level of triglyceride in rats administered o-DCB tended to increase according to the duration of o-DCB administration. 3. The level of ATP in the liver of rats administered o-DCB was significantly lower than that in the liver of control rats. 4. The respiratory control of liver mitochondria of rats administered o-DCB decreased in state3 respiration, and increased in state4 respiration. The respiratory control index of the liver mitochondria decreased after o-DCB was given orally to rats, and it was observed that the longer the period administration was, the lower the respiratory control index was.