This study was conducted to evaluate the potential of 4-vinylcyclohexene (VCH) to induce micronuclei in the bone marrow of mice and rats. Male and female Crl:CD BR (Sprague-Dawley) rats and B6C3F1/CrBR mice were exposed to VCH 6 hr/day for 2 days or for 13 weeks. In the 2-day study, mice were exposed by inhalation to 0, 250, 500, or 1000 ppm, and rats were exposed to 0, 500, 1000, or 2000 ppm. In the 13-week study, mice were exposed to 0, 50, 250, or 1000 ppm, and rats were exposed to 0, 250, 1000, or 1500 ppm. In each study, a separate group of mice was exposed to 1000 ppm 1,3-butadiene (BD) so that a comparison could be made between the two compounds. Likewise, cyclophosphamide was also included for rats as a positive control. Bone marrow was collected from VCH-exposed animals approximately 24 h and 48 h after the final exposure. There were no statistically significant increases in micronucleatedpolychromatic erythrocytes (MN-PCEs) among VCH-treated mice and rats at any dose level or sampling interval at either 2-days or 13-weeks. Also, no statistically significant differences in the polychromatic erythrocytes (PCE) to normochromatic erythrocytes (NCE) ratios were observed in any of the VCH-treated mice and rats compared to air-exposed animals. As expected, both the butadiene-treated mice and the cyclophosphamide-treated rats showed significantly more MN-PCEs than the control animals.