1. The secretion of the adrenal cortex in several mammalian species has been studied by direct analysis of samples of adrenal venous blood, using paper chromatography. 2. All the species examined were found to secrete large amounts of 17-hydroxycorticosterone and/or corticosterone, and some a third compound, provisionally identified as 11-hydroxy- androst-4-ene-3:17-dione. 3. The ratio of 17-hydroxycorticosterone to corticosterone secreted, varied from < 0·05 in rats to > 20 in rhesus monkeys, but did not vary appreciably between members of any one species. 4. In any individual, changes in total secretion rate produced no change in the relative amounts of the different components of the secretion. 5. ACTH increased the secretion rate of all components of the secretion. 6. It is suggested that the species differences observed in adrenocortical secretion are genetically determined, and cannot at present be related to any known differences of adrenocortical function in these species. Observable physiological differences may, however, be discovered, and caution should be exercised in generalizing from the results of ACTH treatment in the rat and rabbit.