An 8 X 8 diallel analysis of locomotor activity related to nest return in mice just prior to eye opening indicated a pattern of dominance toward high activity, with little additive genetic variance. Groups of laboratory-reared wild mice did not differ from each other or from the diallel mean, suggesting little relaxation of selection toward rapid nest return during domestication. In contrast to the nest return situation, an eight-strain triple test-cross analysis of locomotion in a test environment unlikely to be encountered by 11-day-old mice indicated only additive genetic variance, with no evidence of dominance for increased activity. When measured in an ecologically relevant environment, the nature of genetic variation appears to change with age in a manner concordant with what one would intuitively assume to be adaptive behavior at each stage of development.