Male mice of the BALB, ICR and C57B1/6J strains were tested for conspecific aggressive behavior at either 6 or 30 weeks of age. Only the BALB animals were aggressive at 6 weeks of age while both BALB and ICR mice fought at 30 weeks of age. C57B1/6J mice did not show aggressive behavior at either of the time intervals studied. Changes in housing conditions altered the degree of aggressive behavior of the ICR but not the C57B1/6J mice. Whole-brain levels, rate constants (k) and utilization rates (K) of norepinephrine (NE) and dopamine (DA) in these mice were fluorometrically determined. C57B1/6J mice had significantly higher NE levels while BALB mice had higher DA levels, compared to the other strains at both time periods. The NE rate constants and utilization rates for C57B1/6J mice were lower at 6 weeks and higher at 30 weeks than either BALB or ICR mice as was the dopamine rate constant at the 6 week period. There was a tendency for BALB mice to have consistently higher DA utilization rates. These findings are discussed in relation to the catecholamine hypothesis of affective disorders.