To determine the optimum timing of partial hepatectomy (PH) in a previously developed mouse liver micronucleus test (Igarashi and Shimada, 1997), the relation between DNA damage and micronucleus was examined using the in vivo alkaline comet assay and the micronucleus test on the liver of the same individual mouse. Five genotoxic carcinogens, 1-nitropyrene (1-NP) (125 mg/kg), cyclophosphamide (CP) (50 mg/kg), methylmethan sulfonate (MMS) (80 mg/kg), mitomycin C (MMC) (2 mg/kg) and diethylnitrosamine (DEN) (50 mg/kg) were intraperitoneally dosed to each group consisting of 4 male ddY mice. The mice were subjected to PH 3, 8 or 24 hr after dosing of each carcinogen, and comet assay was performed using the removed liver. The regenerated hepatocyte was sampled five days after PH, and the incidence of micronucleus was measured. CP, MMS, MMC and DEN induced DNA damage at 8 and 24 hr after dosing, while 1-NP induced DNA damage only 8 hr after dosing. All five carcinogens induced micronuclei whenever PH was performed. In the case of CP, the peak of DNA damage was 24 hr after dosing and the timing of PH did not remarkably affect the incidence of micronuclei. The other 4 carcinogens showed peak DNA damage at 8 hr and the highest incidence of micronuclei when PH was operated 24 hr after dosing. In conclusion, we are the first to show the relation of induction between DNA damage and micronucleus in the liver from the same mouse, and tentatively showed the optimal timing of PH as 24 hr after dosing.