AbstractPsychomotor development from birth to 18 days of age was observed in 3 lines of mice selected for high (H), medium (M), and low (L) brain weight. Litters were cut to 8 at birth and half of the individuals randomly assigned to a practiced group tested daily; the remainder were assigned to a nonpracticed group and were tested only on selected tests on the day at which their littermates met a predesignated criterion. Significant differences between the lines were found in open field activity, balancing on a rotorod, age of eye‐opening, and edge avoidance. The H‐line was generally more advanced than either M or L. On some measures, righting for example, no strain variation was found. The effects of practice were usually significant, but did not abolish strain differences.Most research on the behavioral effects of genetic variation in the nervous system has been concerned with effects of mutant genes producing major disruption of developmental patterns. Our results indicate that genes affecting the rate and amount of normal brain growth also influence behavioral development. Genes of this type may be responsible for some of the individuality of behavioral phenotype found in natural populati