Nest‐building by male and virgin female wild and laboratory mice kept at a temperature of 2oC was compared with that of controls kept at 23oC. The amount of cotton wool pulled into the cage was recorded over 24 hours. Nest quality was also assessed. The tests of nest‐building were conducted in both cold and warm environments. Cotton pulling was usually at a lower rate in the cold environment, but there was no corresponding decrement in final nest quality. Previous experience in the cold, compared with absence of such experience, resulted in higher scores by wild mice tested in either environment, and by laboratory mice tested in the cold environment. Wild mice that built high quality nests used less cotton wool in the cold than in the warm environment. The tendency for wild mice to pull more cotton wool and build better nests than laboratory mice was more pronounced among cold‐treated animals. Differences in body weight did not account for the differences between wild and laboratory mice.In further experiments nest‐building was observed over 16 days. In the cold environment there was an initial depression of nest‐building by both wild and laboratory mice, followed by a steady improvement over 6 days.Both males and virgin females sometimes made well constructed nests even in the warm environment. In the cold environment the effect of previous cold‐exposure on wild mice was the rapid construction of