A study was made of the action of the adrenocorticolytic drug chloditane (o.p'-DDD) on the content of cytochrome P-450, adrenodoxine and free radicals in the dog adrenal cortex. After a single feeding of 50 mg/kg bw o.p'-DDD EPR signals with ξ-factors 2.25, 1.94 and 2.00 reduced 30-40%. Three-fold feeding of the drug in a dose of 100 mg/kg bw produced a further demonstrable decrease of signals. These changes attest to a possible inhibition of the hydroxylase complex in the adrenal cortex induced by o.p'-DDD.