The isolated hepatocytes of Nile Tilapia(Oreochromis niloticus)were exposed to MC-LR to study the mechanism of microcystin-LR(MC-LR)toxicity.After hepatocytes were exposed to 10,50,150,500 μg/L MC-LR for 24 h respectively,the single cell gel electrophoresis assay(SCGE)was used to investigate the effects on DNA.Induction of DNA strand break in all MC-LR exposed groups was significantly higher than that of the control group.Treatment time-and dose-dependent DNA migration caused by MC-LR and was observed.Flow cytometry(FCM)analysis indicated that the percentage of hepatocyte apoptosis increased in a treatment time-and dose-dependent manner.These results provide first experimental evidence showing the mechanism of MC-LR toxicity at cellular and molecular levels.