Kamyab, Fotherby & Klopper (1968) showed that there were similarities between the in-vivo metabolism in man of 14C-labelled norethisterone (17α-ethynyl-19-nortestosterone) and lynestrenol (3-deoxynorethisterone) and suggested that some of these similarities might be due to the conversion of the latter to the former. The present communication describes such a conversion in vitro. Rabbit liver was homogenized and incubated by the procedure described by Davidson & Fotherby (1965) except that: (1) each flask contained in addition 0·0012 m-NADPH, (2) the incubations proceeded for 16 hr., (3) ethyl acetate was used for extraction instead of benzene and (4) the residue obtained, following evaporation of the ethyl acetate, was submitted to a hexane-methanol partition (Fotherby, Colas, Atherden & Marrian, 1957). For the large-scale incubations, 20 mg. steroid in 0·2 ml. ethanol were added to an homogenate of 20 g. liver. Thin-layer chromatography was carried out using either silica gel G (Merck and