Activity of malate enzymes was studied in human, dog, guinea pig, rat and cattle adrenal glands. The malate enzymes were inhibited in dog adrenal glands after feeding of the animals with 1,1-dichloro-2-(o-chlorophenyl)-2-(p-chlorophenyl)-ethane (o, p-DDD, chloditane, mitotane). The rate of NADP reduction was also decreased if o, p-DDD was added into preparations of dog adrenal gland slices or homogenates in vitro. At the same time, o, p-DDD did not influence on cytosole malate enzymes. Guinea pigs were highly resistant to the adrenocorticolytic action of o, p-DDD; activity of malate enzymes in adrenal glands of the animals was unaltered after intramuscular or per os administration of the drug. In vitro o, p-DDD did not alter the activity of malate enzymes in guinea pig adrenal gland slices or cytosole but reduced the activity in homogenates.