Many behavioural tests in rodents are based on the premise that basal locomotor activity of the animals is similar between the tested groups. The measurement of basal home cage activity is therefore an essential parameter, that should be included in all studies which employ tests of anxiety or cognition. Currently available systems for the assessment of home cage locomotion are often complex and expensive. Here we describe and validate a novel, simple and cost-efficient apparatus for the assessment of basic home cage locomotor activity in rodents. Circadian dark-light activity patterns can be reliably obtained with the home cage activity counter. Furthermore, changes in locomotion induced by novelty or pharmacological treatment were reliably and sensitively detected by the apparatus. Thus, the here presented home cage activity counter can be used for the measurement of basal home cage locomotor activity.