As further evidence that rat adrenals in vitro produce corticoids, the chloroform-soluble substances formed by rat adrenals have been chromatographed on paper in two different solvent systems to compare their mobilities with those of known corticoids. The chromatography was carried out for the shortest possible time compatible with some resolution in order to prevent losses by excessive exposure or by transfer. Eluates of sections of the chromatograms were analyzed for α, β-unsaturated ketone groups and for groups that reduce a tetrazolium derivative (M. and B. 1767). The data may be interpreted as follows: 1. Rat adrenals in vitro produce ultraviolet-absorbing substances with chromatographic mobilities resembling hydrocortisone and corticosterone. Of these substances only 40–70% can be typical corticoids as judged by the reducing: ultraviolet-absorbing ratio. Ultraviolet-absorbing material without reducing properties is also present, most of which has a polarity greater than corticosterone. 2. ACTH, added to rat adrenal glands in vitro, stimulates primarily the production of substances with a polarity similar to that of corticosterone; the production of the other components is also somewhat increased.