The genotoxic effects of steroidal oestrogens are probably brought about by metabolic changes in their phenolic groups accompanied by the generation of quinones and reactive oxygen species. Although non-steroidal oestrogens and related compounds have not been thoroughly investigated for genotoxicity, some of them also contain phenolic groups that could be involved in redox cycling. Therefore, the aim of the present study was to evaluate the possible DNA-damaging effects of the thyroid hormones triiodothyronine (T3) and L-thyroxine sodium salt (T4) and the neurotransmitter noradrenaline (NA) in human sperm using the Comet assay. They were compared with diethylstilboestrol (DES), a steroidal oestrogen, as a positive control. After dose-response studies, doses of 80 microM T3, 80 microM T4, 300 microM NA and 175 microM DES, which produced DNA damage but retained good cell viability, were chosen for further experiments with the antioxidant catalase and the flavonoids kaempferol and quercetin. Since the scavenging enzyme catalase reduced the DNA-damaging effects of T3, T4 and NA, it can be surmised that these compounds under these conditions induced DNA damage mainly via the production of reactive oxygen species. This was further confirmed by the inhibitory responses produced by the flavonoids, which are known to have antioxidant effects. Therefore, the mechanism of mutagenic action of both steroidal and non-steroidal compounds imply the creation of oxidative stress and subsequent DNA damage due to reactive oxygen species and possibly due to reactive hormone derivatives created during their redox cycling.