A new, photocell-equipped, activity meter is described. The motor activity is observed in a large square, "open field", arena (approximately 0.5 m2) suited for activity observations of rats or other similarly sized animals. Horizontal and vertical activity is recorded at two levels (8 x 8 photocells at each level). Information from each photocell, individually fed into a microcomputer, was used to measure total motor activity, activity in the periphery of the arena, forward locomotion, rearing (total and peripheral), and speed of movement. The equipment has been evaluated by studying the effects of six psychoactive compounds, d-amphetamine, apomorphine, phencyclidine, raclopride, haloperidol, and 8-hydroxy-2-(di-n-propylamino) tetralin. In addition, a graph on the distribution of time intervals between successive photocell beam interruptions, and the within- and between-session habituation in normal animals is provided.