Zeranol is a synthetic non-steroidal anabolic agent, with oestrogenic activity. The main metabolites of zeranol in mammals are zeralanone and taleranol. Zeranol and its metabolites are excreted as free compounds and as glucuronide or sulphate tracer. Zeranol is also produced naturally from the mycotoxin zearalenone, which is found in seven strains of Fusaria that have been identified in animal feedstuffs. (4) Therefore, any tissue residues of zeranol may be of natural origin.