A microcomputer-based laboratory system for controlling stimulus presentations and data acquisition in classical conditioning experiments is described. The system comprises an Intel 386/486-based microcomputer and a commercially obtained low-cost counter/timer board with input/output lines for stimulus timing and external device control. A simple, yet versatile custom-designed structured programming language is provided for performing an unlimited number of stimulus configurations and their sequences. In electrophysiological studies, the system can be flexibly connected to computer-controlled signal conditioning systems for the amplification and filtering of multiunit and evoked field potential responses and to high-speed data acquisition systems for sampling and analyzing the responses. The costs of reserving an entire microcomputer for experiment control are well compensated for by the simplicity and efficiency of programming and transportability of the control protocols between different setups and laboratories. Furthermore, a data acquisition and analysis system most suitable for the aims of a research project can be selected.We have developed a control system that flexibly performs the timing and delivery of experimental stimuli in classical conditioning experiments. It offers commands for event timing, external stimulus device control, data acquisition system synchronization, and video screen display control. Our system may be a useful investment for experimenters using a wide variety of equipment and systems for behavioral and electrophysiological data collection. It is particularly suitable for experiments requiring continuous AID conversion with high sampling ratesfor example, when multichannel, multiunit, or evoked field response recordings are performed.Although in classical conditioning experiments only a limited set ofstimuli are used and the stimuli are presented independently of the subject's behavior, there is a great variability in their relations and timings (e.g., Gormezano, Kehoe, & Marshall, 1983;Rescorla, 1988). The present experiment control system was therefore designed to be versatile enough to adapt to a wide variety of experimenThis research was supported by the Finnish Academy. Address correspondence to M. Penttonen,