Either minced or homogenized preparations of head kidney tissue from Tilapia mossambica were incubated with different radioactive precursors. From acetate–l–14C only a small yield of progesterone was obtained. Cholesterol–4–14C was actively metabolized into four major unidentified compounds, observed also after incubation with Tilapia spleen, but its contribution to corticosteroidogeneis could not be demonstrated. Conversely, rat adrenal tissue promptly converted cholesterol–4–14C into progesterone, corticosterone, and 11–dehydrocorticosterone. With minced head kidney, progesterone–4–14C gave rise to 17α–hydroxyprogesterone, 11–deoxycortisol, cortisol, cortisone, 11–deoxycorticosterone, corticosterone, 11– dehydrocorticosterone, 11β–hydroxyprogesterone, and 21–deoxycortisol. Homogenization caused a greater accumulation of those metabolites proximal to progesterone and a reduction in the total steroid profile. Aldosterone formation did not occur from progesterone–4–14C or from corticosterone–4– 14C. The latter was mostly oxidized to 11–dehydrocorticosterone and did not undergo 17α–hydroxylation to cortisol. The lack of a shunt of 17–deoxycorticosteroids toward cortisol production was confirmed by the complete segregation of the 14C and 3H labels after incubation with equimolar amounts of 17α–hydroxyprogesterone–4–14C and 11–deoxycorticosterone–l, 2–3H. Radioactive metabolites were always identified by their isopolarity and isomorphicity with standard compounds. (Endocrinology91: 450, 1972)