Single stranded DNA breakage induced by lead nitrate in mice has been studied in vivo using alkaline single cell gel electrophoresis (comet assay). Mice were administered orally 0.7, 1.4, 2.8, 5.6, 11. 2, 22.4, 44.8 and 89.6 mg/kg body weight of lead nitrate and the assay was performed on whole blood at 24, 48, 72 h, 1st and 2nd week. Significant increase in mean tail-length of DNA was observed at all time intervals after treatment with lead nitrate when compared to controls. The mean tail-length did not show a dose-related increase and the elevation in the mean tail-length was of a fluctuating type. Increase in mean tail-lengths clearly gives evidence that lead nitrate causes DNA damage effectively. The study indicates that the alkaline comet assay is a sensitive and rapid method to detect DNA damage caused by heavy metals.