The in vitro metabolism of progesterone-4-14C and 11-deoxycorticosterone-4-14C acetate in a rat adrenocortical carcinoma, 494 and 494-H, has been studied and compared with adrenal tissue from normal rats. An accumulation of deoxycorticosterone in the tumour tissue with an accompanying decrease of corticosterone and aldosterone, indicates a blockade in the 11-hydroxylation of deoxycorticosterone. A difference in the distribution of radioactive metabolites is observed between incubation experiments with Tumour 494 and 494-H. These findings indicate a correlation between the metabolic activity and the physiological findings in the host.