An experimental control system was designed to evaluate the effects of toxic substances. This system consisted of a personal computer (Apple II), shuttle boxes, and their controller. The shuttle box (33 X 20 X 53 cm) was divided into two compartments of equal size with a hurdle (8 cm). An infrared beam device was set up 8 cm from each side of the hurdle and 5 cm above the floor to detect the occupied compartment. Signals from detectors were put into the Apple II through a peripheral interface adapter unit. Since the control program was written in assembly language, which could operate at high speed, it was possible to conduct experiments using four shuttle boxes simultaneously. During the execution of the experiments, the timer unit generated pulses 100 times/sec. Every pulse was counted and used to measure a response latency at intervals of 10 msec. The application of a computer-controlled infrared beam device made it possible to detect the location and operant responses of a subject mechanically from moment to moment and to construct an extremely flexible experimental control system.