The aim of this study was to investigate the in vivo genotoxic effects of carvacrol and thymol in bone marrow cells of rats. In the present study, both carvacrol (10, 30, 50, and 70 mg/kg b.w.) and thymol (40, 60, 80, and 100 mg/kg b.w.) significantly induced the structural and total chromosome abnormalities (CA) for all treatment periods (6, 12, and 24 h) when compared with control in bone marrow cells of rats intraperitonally administered. Both carvacrol and thymol showed similar effects with the positive control urethane on induction of the percentage of structural and total CA at the highest concentrations except the effects of carvacrol for 6 h treatment (70 mg/kg b.w. and 100 mg/kg b.w., respectively). In addition, carvacrol induced the numerical CA at all concentrations when compared to control and at two highest concentrations (50 and 70 mg/kg b.w.) when compared to solvent control. Thymol also induced the numerical CA especially at the highest concentration (100 mg/kg b.w.) for all treatment periods. It was shown that there was a dose-dependent effect on induction of structural, numerical and total CA for both carvacrol and thymol. Carvacrol and thymol decreased the mitotic index (MI) in all the concentrations and treatment times when compared with control. Carvacrol showed the similar effects with EC on decreasing the MI at 70 mg/kg b.w. for 6 h, at 30 and 50 mg/kg b.w. for 12 h and at all concentrations for 24 h treatment periods. Thymol also showed a similar effect with urethane (ethyl carbamate, EC) on decreasing the MI at 60, 80, and 100 mg/kg b.w. for 6 h and at all concentrations for 24 h treatment periods. Test substances decreased the MI in a dose-dependent manner. (C) 2008 Wiley Periodicals, Inc. Environ Toxicol 23: 728-735, 2008.