1. The metabolism of [4- 14 C]pregnenolone to androst-16-enes has been studied in short-term incubations of boar testis tissue. With fresh tissue androsta-5,16-dien-3β-ol (8%) and 5α-androst-16-en-3β-ol (2%) were formed. Tissue that had been stored at −20°C was still capable of metabolizing pregnenolone to androsta-5,16-dien-3β-ol. 2. NADPH was essential for the formation of androsta-5,16-dien-3β-ol from pregnenolone; NADH had less activity and ATP was not necessary for the reaction. 3. [4- 14 C]Androsta-5,16-dien-3β-ol, prepared biosynthetically from [4- 14 C]pregnenolone, was shown to be converted by boar testis preparations into androsta-4,16-dien-3-one (31%) if NAD + was present or into 5α-androst-16-en-3β-ol (4%) if NADPH was present. 4. 17α-Hydroxyandrost-4-en-3-one and 3β,17α-dihydroxypregn-5-en-20-one were considered as possible precursors for androst-16-ene formation, but both were shown to be ineffective. 5. No radioactivity was incorporated into androst-5-en-3β-ol used to trap any corresponding 14 C-labelled compound formed from [4- 14 C]pregnenolone.