Running wheels are frequently used in behavioural and physiological experiments. The function of wheel-running activity in laboratory animals is controversial. In the present long-term study, the influence of this activity was evaluated in male golden hamsters over a period of 52 weeks. Four months after the start of the experiment, hamsters with access to running wheels were significantly heavier than those without these wheels. In addition, food consumption nearly doubled. The absolute values of fat-free mass (FFM), total body water (TBW) and crude fat mass (CFM) increased. However, in contrast to these absolute differences, the relative values were never different and general body composition was therefore unaffected by running-wheel activity. Different organ masses were established for absolute values of kidneys, testes and epididymis; possible effects on reproduction are discussed. The present data indicating improved physical condition leads to the assumption that a running wheel is a useful enrichment, enhancing animal welfare in the golden hamster.