SUMMARY: The action of different steroid hormones on the oxygen consumption of adrenal cortex slices in vitro was investigated. Deoxycorticosterone (DOC), testosterone and dehydroisoandrosterone, and 17-hydroxycorticosterone, were found to stimulate the oxygen uptake rate, while cortisone, progesterone, pregnenolone, androsterone and cholesterol were inactive. Pretreatment of the slices with DOC, testosterone or ACTH inhibited the action of ACTH or DOC added subsequently. The significance of the direct action of different steroid hormones on the energy metabolism of the adrenal cortex is discussed.