Zeranol (7-alpha-zearalanol or alpha-ZAL) metabolism in vitro by subcellular fractions (microsome and cytosol) from lamb livers was investigated. The use of a high performance liquid chromatography (HPLC) technique capable of resolving epimers, revealed that when nicotinamide adenine dinucleotide (NAD) was added as a co-factor to metabolic mixtures, the oxidized form, zearalanone (ZAN) was the major metabolite, although a small amount of 7-beta-zearalanol (beta-ZAL) was also produced. In order to confirm beta-ZAL as a product of ZAN reduction, the metabolism of the latter, by microsome and cytosol in the presence of NADH as co-factor, was investigated. The results obtained revealed that both alpha-ZAL and beta-ZAL were present at the end of the incubation, the former at a higher concentration than the latter. When NAD and NADH were added as cofactors to incubation mixtures containing alpha-ZAL, the production of beta-ZAL was increased as a consequence of the higher ZAN reduction in the presence of the reducing co-factor. Nevertheless, ZAN remained the major metabolite produced from alpha-ZAL by both the subcellular fractions investigated.