Human adrenals from twin female fetuses (10 cm crown-rump length) were minced and incubated in a similar manner with both pregnenolone-7[alpha]-H3 and progesterone-4-C14. Radioactive products were identified by isotope dilution. The patterns of steroid metabolism in these adrenals were similar but not identical. 11[beta]-Hydroxyandrostenedione was a major product of both pregnenolone and progesterone metabolism. Cortisol was formed in detectable amounts from progesterone only. A large amount of unidentified, polar radioactivity was found in the extract of the incubated material. Evidence is presented for the probable monozygocity of the fetuses.