The metabolism of 19-nor[4-14C]testosterone has been studied in the equine castrate. Following XAD-2 extraction of aliquots of the 0-24 h urine samples, the glucuronic acid and sulphate conjugates were separated by Sephadex LH-20 column chromatography. After hydrolysis of the conjugates, the neutral phase I metabolites of 19-nortestosterone were extracted, purified and identified by g.l.c.-mass spectrometry. In phase I metabolism stereospecificity was observed in the reduction of the A-ring with the formation of the 5 alpha, 3 beta-isomers of estranediol. Epimerization at C-17 and hydroxylation at C-16 were the other major pathways. In phase II metabolism the C-17 alpha steroid epimers were predominantly conjugated with glucuronic acid and the C-17 beta epimers with sulphuric acid. One animal showed a slight variation in metabolism with a tendency towards the formation of polar metabolites.