Because a unilateral lesion of the dopaminergic nigrostriatal system has been found to induce rotational behavior in primates, as well as in rodents, we have designed and constructed a rotation monitoring device suitable for non-human primates. Animals are connected via a tether to a continuous-rotation potentiometer which provides a position-dependent output voltage. This voltage is interfaced via an analog-to-digital converter to an IBM PC. The rotation program then calculates the direction and amount of any turning, as well as total activity, and this data is recorded on diskfile for later analysis. Because the system can operate around the clock unattended, data can be collected over long experimental periods, and detailed circadian analysis of rotation and total activity can be made. The design is also applicable to smaller animals, including rodents, and has many advantages over previously available rotometer designs.