Recent evidence has indicated that the intensive properties of the operant response may provide information not apparent from rate data alone. In the present study 11 rats were trained on a simple operant discrimination and the duration of each response was recorded. Mean S[DELTA] durations were consistently longer and more variable throughout training than average S[DELTA], or reinforced, durations. Moreover, the duration of the first response in an SD or S[DELTA] trial provided a reliable index of discriminative behavior. This duration index of discriminative behavior showed differential performance developing as much as 3 days prior to its indication by a conventional response-rate index.