A crude mixture of capsaicinoids (CAPs) from Capsicum frutrescens, capsaicin (C) and dihydrocapsaicin (DC) but not 4-methyl-dihydrocapsaicin (MC), a synthetic homolog of DC, were mutagenic in the V79 assay. Oxidative metabolism of C and DC by microsomes or ferricyanide yielded a dimer of C or DC. Based on the lack of mutagenicity of MC, the mutagenicities of C and DC and the supporting chemical data, a mechanism of action involving an intermediate phenoxy radical is proposed.