Adult male and female woodmice (Apodemus sylvaticus L.) were isolated for 4-6 days in actographs. Time spent in the nest box, eating, drinking, locomotor activity in the enclosure ,and wheel-running were automatically recorded. The individual breeding cages were also equipped with a wheel. The immediate reaction following the introduction in an actograph, at about 11.00 a.m., lasted for 30-120 min; locomotor activity and wheel-running was higher in the male than in the female group. During the first night, the mice were more active in the whole enclosure and ran less in the wheel than during the following nights. This was more evident in the female group, but both groups showed similar activity from the third day, and expressed more wheel-running than when in the breeding cage. When brought back into the breeding cage, the mice's wheel-running activity strongly decreased on the first night, and was even lower in the female group than before testing. In the actographs, increasing the accessible surface around the wheel led to a transitory drop in wheel-running for 1-2 days. Cleaning the breeding cage induced a similar but less pronounced change, On the whole, the novelty of a situation depressed wheel-running and increased locomotor activity for a few days. This reaction was more important in the female than in the male group, although the males showed a greater amount of activity as an immediate response to environmental changes., (Copyright © 1979. Published by Elsevier B.V.)