Selected enzymatic systems can efficiently produce a product in the electronically excited triplet state. Earlier, only the formation of electronically excited singlet species was known. The formation of triplet species has been demonstrated with both normal substrates/metabolites and with xenobiotics, even at the cellular level. Triplet excited species have intrinsically much longer lifetimes than excited singlets, whereby they can be potentially important agents for normal and/or deleterious processes, including mutagenesis. Enzymically generated triplet species can damage DNA, even when protein coated, as in the case of the lambda-phage of Escherichia coli. Some evidence of damage by triplet species has also been reported for intact cells. Triplet excited species may produce their effects through type I and/or type II dark photosensitization, that is, the events may be started by H abstraction and/or singlet oxygen/superoxide ion production. The induction of lipid peroxidation, with concomitant clastogenic effects, appears to be of special importance.