When 11-deoxycorticosterone was added to the incubation medium of isolated, quartered rat adrenals in the absence of ACTH, 21% was converted to corticosterone. 10% to the 20[forward arrow]18-hemiketal of 18-hydroxy-11-deoxycorticostorone and 3% to material with a mobility relative to corticosterone (RB) of 0.10 in the toluene-propylene glycol system. The corresponding figures obtained with progesterone were 11%, 4% and 2% respectively. The addition of 18-hydroxyproges-terone caused an increase in the production of the 20[forward arrow]18-hemiketal of 18-hydroxy-11-deoxycorticosterone, which accounted for 6% of the added steroid; it also caused the formation of an unidentified compound of RB 1.13 that did not reduce tetrazolium and yielded an absorption maximum in methanol at 240 m[mu]. Corticosterone only increased the formation of material with RB 0.10. The recovery of aldosterone was not affected by any of the added steroids. The conversion of added steroids was either affected or inhibited by ACTH.