Rosiglitazone (RSG), a member of the thiazolidinedione class of antidiabetic agents, improves glycemic control by increasing insulin sensitivity. The therapeutic mode of action of RSG involves its activity as a highly selective and potent agonist for peroxisome proliferator-activated receptor-gamma. Although other drugs in this class have displayed unacceptable hepatotoxicity, RSG was approved for human use. The package insert indicates that RSG has minimal genotoxicity, but information on the genotoxicity of RSG is not available in the published literature. In this study, we used the single cell gel electrophoresis (SCGE)/Comet assay to investigate the DNA damage in peripheral blood and liver cells of rats treated with RSG. Sixteen male Sprague-Dawley rats were randomly distributed into four groups, and dosed daily by oral gavage with 0.0, 0.5, 1.0, and 2.0 mg/kg/day RSG. The rats dosed with 2.0 mg/kg/day RSG received an similar to 10-times the area under the curve concentration of the maximum recommended human daily dose. After 14 days of treatment, the rats were euthanized, and peripheral blood and liver were collected and. processed for the Comet assay. A dose-dependent increase in DNA damage (as assessed by % tail DNA and Olive Tail Moment) was observed in the hepatocytes of RSG-treated groups, with significant increases detected between rats treated with all the doses of RSG and the control, and between rats treated with different RSG doses (P < 0.05 - P < 0.0001). In contrast, DNA damage was detected in peripheral blood lymphocytes only in rats treated with the higher RSG doses (1.0 and 2 mg/kg/day). Taken together, the data indicate that RSG is able to induce primary DNA damage in rats, with greater damage being detected in liver cells than lymphocytes.