A new schedule of reinforcement was used to maintain key-pecking by pigeons. The schedule reinforced only pecks terminating interresponse times which occurred least often relative to the exponential distribution of interresponse times to be expected from an ideal random generator. Two schedule parameters were varied: (1) the rate constant of the controlling exponential distribution and (2) the probability that a response would be reinforced, given that it met the interresponse-time contingency. Response rate changed quickly and markedly with changes in the rate constant; it changed only slightly with a fourfold change in the reinforcement probability. The schedule produced stable rates and high intra- and inter-subject reliability, yet interresponse time distributions were approximately exponential. Such local interresponse time variability in the context of good overall control suggests that the schedule may be used to generate stable, predictable, yet sensitive baseline rates. Implications for the measurement of rate are discussed.
