The present experiment was carried out to investigate whether X and Y chromosome dissociation in the primary spermatocytes of mice can be used as an in vivo assaying system that detect environmental mutagens. For this purpose, alkylating agents (EMS, MMS and MMC), which are strong mutagens, were administered to ICR male mice 12-15 weeks old. The mean frequencies of previously dissociated X-Y chromosomes and autosomes of the control group were 7.34-7.45% and 0.92-1.04%, respectively. The frequencies of X-Y dissociation in the mutagen-treated groups with 10.0 mM EMS and 5.0 mM MMS were about 3.3-4.6 times higher than that in the control group, but there were no significant differences in dissociation of autosomes in both the control and the mutagen-treated groups. These results suggest that X-Y dissociation in the primary spermatocytes of mice can be used as an in vivo short-term assaying system for environmental mutagens.