The adrenalytic activity of mitotane (o,p'-DDD) has made it useful in the treatment of adrenocortical carcinoma and Cushing's syndrome. In support of a study to develop mitotane analogs as more effective therapeutic agents and as a basis for understanding the toxicity of related compounds in the adrenals, the biotransformations of o,p'-DDD in adrenocortical homogenate preparations have been studied and compared with those of its m,p'- and p,p'-isomers. Aliphatic oxidation to the corresponding acetic acid derivative, o,p'-, m,p'- or p,p'-DDA, was the major transformation for all the preparations. In the comparisons of the DDD isomers, the order of both DDA formation and apparent covalent binding was o,p'- > m,p'- > p,p'-DDD. There was also evidence for alpha-hydroxylation at the benzylic carbon with subsequent loss of water to form ethylene derivatives. This was a minor pathway for o,p'-DDD, but was the major pathway for the other two isomers. Thus, while the total yields of metabolites of o,p'- and m,p'-DDD were similar and at least twice that of the p,p'-isomer, their distribution of metabolites differed significantly. The effects of the three isomers on cell growth and cortisol production with the human adrenocortical carcinoma cell line, NCI H-295, followed the same order as their DDA formation and tissue binding. It is proposed that hydroxylation by the adrenal cortex at the beta-carbon leads to an adrenalytic effect, whereas hydroxylation at the alpha-carbon would represent an alternate deactivation pathway.