Four age groups of C57BL/6J mice (2.2, 6.2, 12.0, and 23.3 months) were subjected to either immobilization or handling (control) procedures. Open field behavior was observed before and after experimental treatments and plasma corticosterone levels were assessed 11 days following the immobilization or handling procedures. Eleven days following immobilization elevated corticosterone levels were observed for all but the 12.0 month group of mice. No behavioral effects were observed for the experimental groups, although both locomotor activity and exploratory behavior declined with advancing age. The age-related decrease in activity was entirely accounted for by scores on the initial open field test. Exploratory behavior was observed to be a more complex function of both age and experience.