This report describes a new kind of actometer for recording the behavior of rodents or other small animals. The instrument, a force-plate actometer, uses a stiff, low-mass horizontal plate coupled to four supporting force transducers positioned at the corners of the plate. When an animal moves on the plate, its movements are sensed by the transducers whose signals are processed by computer to yield measurements of a wide range of behaviors or behavioral attributes, such as locomotor activity, rotation around the center, whole-body tremor, and amphetamine-induced stereotypies. Spatial resolution is less than 1 mm, and temporal resolution is 0.02 s. Sample data were presented comparing the locomotor activity of CD-1, BALB/c, and C57BL/6 mice before and after treatment with D-amphetamine sulfate. Rotational behavior was recorded in an amphetamine-treated rat that had sustained a unilateral 6-hydroxydopamine-induced lesion of the nigrostriatal system. In the C57BL/6 mouse, harmaline-induced tremor was quantified. With rats as subjects, the force-plate actometer was used to quantify amphetamine-induced stereotypies, to demonstrate the development of sensitization to amphetamine's effects, and to quantitate the consistent 11-12 Hz rhythmicities that underlie the sterotypies. The performance of the force-plate actometer was compared with that of a variety of instruments reported in the literature on behavioral instrumentation. Finally, potential applications in neuroscience research other than those illustrated in this report were discussed.