The effect of shock on locomotor activity was evaluated in three strains of mice (A, DBA/2 and C57BL/6) after treatment with scopolamine (1.0 mg/kg) and d-amphetamine (10.0 mg/kh). The effectiveness of either drug in incrasing locomotor activity was strain dependent. Both drugs eliminated behavioral suppression induced by shock, and in A and DBA/2 mice shock augmented the locomotor stimulation induced by d-amphetamine. In another experiment the behavior of the 6 F1 hybrids was examined in relation to the parent strains. It was observed that locomotor activity in the F1's could resemble that seen in one parent in the saline condition, but the other parent after treatment with d-amphetamine. Similarly, the F1 behavior in the amphetamine condition was not predictive of the behavior seen after shock plus amphetamine. The results suggest that general activity, locomotor activity after amphetamine treatment, and responsiveness following shock in amphetamine-treated mice are mediated by different genetic mechanisms.