Mutagenic activity was monitored during the time course of the vinification process of a Portuguese red wine using the Ames assay. Dependence upon faecalase treatment for detection of mutagenicity was evaluated concurrently with HPLC analysis of the flavonoids rutin and quercetin. Rutin (quercetin-o-3-rutinoside) is liberated from the grapes during the first 10 days of the process and is hydrolysed as the vinification process proceeds. This explains the levels of the free quercetin in the wine and mutagenicity in the absence of faecalase treatment. Mutagenicity in the presence of faecalase during the first 26 days of the process correlates with the levels of rutin, and in the absence of faecalase the highest mutagenic activity detected (160th day) coincides with the highest level of free quercetin. Our results do not rule out the possibility that other mutagens are present in wines, in particular oxidative-type mutagens.