Locomotor activity of 2,140 four-day-old inbred, hybrid, and wild mice was measured. Consonant with the prediction that high rates of locomotor activity would be maladaptive in infant mice, hybrids were less active than inbred lines. A triple test cross analysis indicated low heritability and nearly complete dominance toward low activity. Mice from wild stock were even less active than hybrids, which suggests that selection pressures for low infantile activity have relaxed during laboratory domestication. A test is described for estimating changes in selection pressures resulting from laboratory rearing.