Recent studies performed with crude extracts of mouse tissues showed that the activity of DNA-polymerase iota (Pol iota) can be detected only in brain and testis extracts. To assess whether the activity of Pol iota is associated with animal behavior, we determined Pol iota activity in brain extracts of mice of two lines sharply differing in aggressiveness (RSB and RLB). We found that Pol iota activity in the mice with aggressive behavior was three times higher than in the less aggressive mice. The possible relationship between the activity of Pol iota and animal behavior is discussed.