The metabolism, in vitro, of isotopically-labelled pregnenolone and progesterone by foetal and adult sheep adrenals has been investigated. Both substrates were almost completely metabolized by the adult tissue, whereas, in the case of the foetus, only pregnenolone showed extensive metabolism. The adult adrenal converted pregnenolone mainly to cortisol, corticosterone and 11-deoxycortisol, whereas corticosterone was the major product from progesterone. The foetal adrenal, on the other hand, yielded mainly progesterone from pregnenolone with only a small conversion to the corticosteroids. The substrate, progesterone, was transformed, in the main, to 11-deoxycorticosterone. No 3β-hydroxysteroid sulphokinase-transferase activity was demonstrated in either tissue. The importance of these findings and the probable pathways involved in the formation of the biosynthetic products are discussed.