Three pigeons pecked for food on a synthetic variable-interval schedule of reinforcement that had two independent parts: a variable-interval schedule that arranged a distribution of interreinforcement intervals, and a device that randomly assigned each reinforcement to one of 10 classes of interresponse times. The frequencies of reinforcement for the 10 classes of interresponse times were systematically varied, while the overall frequency of reinforcement was held within a comparatively narrow range. The 10 classes extended either from 0.1 to 0.6 sec in 0.05-sec intervals, or from 1.0 to 6.0 sec in 0.5-sec intervals. In the former case, some control by reinforcement was obtained, but it was weak and no simple relationships were discernible. In the latter case, the relative frequency of an interresponse time was a generally increasing function of its relative frequency of reinforcement, and two simple controlling relationships were found. First, the function relating interresponse times per opportunity to reinforcements per opportunity was, over a restricted range, approximately linear with a slope of unity. Second, when all 10 classes of interresponse times were reinforced equally often, the relative frequency of an interresponse time approximately equalled the relative reciprocal of its length.A long-range goal of the experimental analysis of behavior is the development of methods that precisely control the probability with which a response occurs at a specific moment in time. The attempt to reach this goal has led to the use of response rate and interresponsetime distributions as measures of the probability of occurrence of that response. Unfortunately, classical schedules of reinforcement do not control all of the contingencies that are believed to determine response rate and interresponse-time distributions (Anger, 1956;Blough and Blough, 1968;Jenkins, 1970;Morse, 1966 which interresponse times are emitted depends on wlhich interresponse times are reinforced, and at the same time, which interresponse times are reinforced depends on which interresponse times are emitted, i.e., they depend on the organism. This interaction vastly complicates the search for functional relationships between response rate and frequencies of occurrence of interresponse times on the one hand, and reinforcement parameters on the other hand, in variable-interval schedules. Anger (1954)