Microcystin-LR (MCYST-LR) and nodularin (NOD) produced by cyanobacteria are potent and specific hepatotoxins. The induction of free-radical formation, reduction of glutathione levels and induction of DNA damage are three major events found in rat hepatocytes treated with these hepatotoxins. However, the mechanism of MCYST-LR- and NOD-mediated induction of oxidative DNA damage has not been fully elucidated. The objective of this study was to determine whether MCYST-LR and NOD increase the formation of a DNA oxidative damage marker such as 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dG) in vitro in primary rat hepatocytes and in vivo in rat liver cells. Rat hepatocytes were exposed to MCYST-LR or NOD at low doses (2 and 10 ng/ml), at which there is no evidence of morphologically apparent cytotoxic effects, as well as an induced dose- and time-dependent formation of 8-oxo-dG. Moreover, MCYST-LR treatment of rats (50 microg/kg, ip) resulted in a significant increase of 8-oxo-dG in liver DNA, at 24 h after treatment before decreasing at 48 h. However, NOD-induced DNA damage was increased both at 24 and 48 h, in contrast to the MCYST-LR-induced effect. The effects on this oxidative DNA damage marker indicates that MCYST-LR and NOD do evoke oxidative stress, which may contribute, at least in part, to their liver toxicity and carcinogenicity during long-term exposure.