Benzene and toluene have been widely employed as industrial solvents in Korea. However, they have recently been identified as cytogenetic toxic agents. This study is to observe the cytogenetic toxicities of benzene and toluene singularly and combined. The following concentrations of solvents were administered twice intraperitoneally to Sprague-Dawley rats: low concentration (11mg/Kg benzene and 108.75mg/Kg toluene), middle concentration (220mg/Kg benzene and (217.5mg/Kg toluene), and high concentration (440mg/Kg benzene and 435mg/Kg toluene). A low concentration represents the short term exposure limit of industrial workers. To examine the cytogenetic effects of the above solvents, the micronucleus test and the metaphase analysis were conducted followed by a statistical analysis based on non-parametric methods such as the Kruskal-Wallis multi sample test and the distribution free multiple comparison test A low concentration of benzene did not produce significant changes, however the two higher concentration of benzene showed clear signs of cytogenetic toxicities of bone marrow cells (i.e., the micronucleus occurrence rate and the chromosomal aberration rate were increased and the polychromatic erythrocyte percentage was decreased). While a low concentration of toluene produced no significant changes, the two higher concentrations of toluene showed similar signs of cytogenetic toxicities to bone marrow cells but to a somewhat lesser degree than benzene. When benzene and toluene were administered simultaneously at the two higher concentrations in order to observe their combined effects, all three signs of cytogenetic toxicities of bone marrow cells were decreased to a greater degree than the administration of benzene only. However, there were no significant reduction in the cytogenetic toxicities when benzene and toluene were simultaneously administered at low concentration. The above results showed that higher concentration of benzene and toluene displayed cytogenetic toxicities but showed competitive inhibition when they were administered simultaneously. However, there were no significant changes at low concentrations.