To test the effects of prior experience with nest building, we measured adult mice of three inbred and one outbred stock for 4 days on 7 consecutive weeks. A tendency to increase the amount of cotton used between week 1 and week 2 was no greater than effects of environmental fluctuations from week to week. Strain and sex differences were significant, and inbred-strain rank orders remained constant across time. To study developmental effects, we subjected independent samples of four inbred stocks and one outbred stock at each of six ages to one 4-day test. There was a large increase in nesting between mice 30 and mice 40 days of age, and another increase between 72.5 and 90 days. Strain and sex differences did not change significantly after 40 days of age. Our results show that previous experience should not alter conclusions about genetic influences on nesting behavior, which are fully expressed by 40 days of age.