The genotoxic effects of mercury chloride and lead acetate were evaluated in vivo using the micronucleus (MN) assay on acridine-orange (AO) stained peripheral blood erythrocytes, gill and fin epithelial cells of Carassius auratus auratus. Fish were exposed to three different concentrations of mercury chloride (MC) (1 mu g/, 5 mu g/L and 10 mu g/L) and lead acetate (LA) (10 mu g/L, 50 mu g/L and 100 mu g/L) for 2, 4 and 6 days. A single dose of 5 mg/L cyclophosphamide was used as a positive control. In addition to micronuclei, nuclear buds (NBs) were assessed in the erythrocytes. The ratio of polychromatic and normochromatic erythrocytes (PCE/NCE) in peripheral blood was also evaluated to assess cytotoxicity. MN frequencies in all three tissues were elevated in fish exposed to both LA and MC. However, NBs showed different sensitivity to metal treatments. MN frequencies in both control and treated fish were highest in gill cells and generally lower in erythrocytes and fin cells. PCE/NCE rations decreased in relation to MC and LA treatments. The results of this study indicate that LA and MC have genotoxic and cytotoxic damage in fish and confirmed that AO staining is a suitable technique for in vivo MN test in fish. (C) 2007 Elsevier Ltd. All rights reserved.