Swiss-Webster stock male and female mice were tested for running activity after a 20 mg/kg dose of levorphanol. The best runners and worst runners were then bred. Within each litter, brother-sister pairs with the highest and lowest running activity after a test dose of levorphanol (20 mg/kg) were segregated and bred. This procedure was followed to the F4 generation. Running activity, analgesia and brain catecholamines were studied. The results showed that the "non-running" (NR) trait segregated at the F1 generation, while the "running" (R) trait diverged more slowly. However, by the third generation, the R mice differed significantly from the F0 stock mice. NR mice also did not show locomotor activity when tested with amphetamine. While the NR mice showed reduced running response to levorphanol they were significantly more sensitive than the F0 stock to the analgesic effect. Measurement of brain catecholamines showed no difference between R and NR mice in dopamine or serotonin, but did show a significant increase in norepinephrine in the NR strain.