SUMMARY: Ovaries excised from 5- and 7-month-old laying hens were homogenized. The homogenates were incubated separately with 4-14C-labelled pregnenolone, progesterone, 17α-hydroxyprogesterone, dehydroepiandrosterone, androstenedione, testosterone, oestrone and oestradiol, and [7-3H]17αhydroxypregnenolone in the presence of NAD+ or NADPH under aerobic conditions. The homogenates synthesized progesterone, 17αhydroxyprogesterone, 17α-hydroxypregnenolone, dehydroepiandrosterone, androstenedione, testosterone, pregnanedione, 17α-hydroxy-5β-pregnane3,20-dione, 5β-androstane-3,17-dione, 19-hydroxyandrostenedione, 19hydroxytestosterone, oestrone, oestradiol and other minor metabolites. However, the rates for the conversion of androstenedione to oestrone and that of testosterone to oestradiol were low, and after 120 min incubation, the yields were 3·0 and 1·2%, respectively. The results indicate possible involvement of metabolic pathways both from Δ5-C21 steroid to Δ4-C21 steroid and from Δ5-C21 steroid to Δ5-C19 steroid in the formation of androgen by the ovary of the domestic fowl.