Abstract: An improved animal activity monitor of the type in which an animal is positioned in an arena having an array of two perpendicular sets of parallel light beams above the arena for detecting the animal position. In the improvement, transparent walls parallel to the beams subdivide the arena into areas, diagonally related ones of which may be used as subarenas for testing smaller animals. Each subarena has a unique pair of perpendicular beams which cannot be blocked by any other animal. The light beam array for each subarena is separately detected and processed in the manner formerly done for the entire arena. Individual switches associated with each arena or subarena or software routines to detect the presence and absence of an animal in each arena are used to enable or disable the collection of data from the arena or subarena. Each animal may be, in sequence, prepared and then inserted into the arena or subarena.