A Behavioral Pattern Monitor (BPM) is described which is designed to assess the spatial and temporal sequences of the locomotor movements, investigatory holepokes, and rearings of rats. The system records these behavioral responses with 0.1 sec resolution in time and 1.5 inch resolution in space, and permanently stores all the resulting data. The sequences of these responses may then be displayed on a video terminal or on paper and are also available for the calculation of a variety of descriptive statistics. Studies are described in which rats were tested repeatedly without any pharmacological treatments or in single test sessions following the administration of saline or one of five stimulant drugs. A variety of descriptive measures of the temporal or spatial patterning of the animals' behavior are described and applied to the data resulting from the studies of the various stimulants. It is concluded that the combination of these measures enables distinctions to be made among these drugs which cannot be made on the basis of measures of the amount of locomotor activity.