AbstractThe wheel‐running activity of caged mammals has been misinterpreted for many years as a measure of ‘general activity’. A review of the literature and recent experimental evidence suggests that this behaviour has a far more specific function for the animal, and that its major and invaluable experimental use lies as both a field and laboratory tool for the studies of particular forms of migration. This new interpretation allows a greater understanding of the motivations underlying this widely‐monitored behaviour.SummaryIt is apparent that the nature of the response to an activity‐wheel has been greatly misunderstood for many years. The behaviour appears to be far more specific than previously thought, reflecting a type of inherent response which is not evident when activity is recorded by other methods. This report indicates that the wheel is used by a caged animal when the individual is motivated to reach an unattainable resource. This results in an urge to travel, either to remove itself from the immediate area, or to search for specific resources. It is proposed that when the goal is perceptually not present, the activity wheel is a specific monitor of ‘exploratory migration’, and reflects the urge to collect information about the location of resources. It implies, therefore, that the use of the activity‐wheel as a simple measure of ‘general activity’ should cease; the major future uses of this particular activity recording device should be in the studies of the daily, ontogenetic and seasonal variation in the incidence of exploratory migration, and the influence upon it of other environ‐ mental factors, as well as a method of investigating goal‐orientation, both in the field and the laboratory. This new interpretation provides a more precise explanation of what is being measured in wheel‐running experiments, and should result in a more specific use of wheel‐