Computer modeling was used to investigate the extent to which response rates under variable-interval and variable-ratio schedules are compatible with a simple process of interresponse time (IRT) reinforcement, as argued by Peele, Casey, and Silberberg (1984). Their computer model was duplicated, as well as its principal result of a large response rate difference between the interval and ratio ... [Show full abstract] schedules. After their model was run under a variety of interval and ratio schedules, it was found that the response rates produced did not exhibit patterns of sensitivity to schedule parameter variation found experimentally. Furthermore, the model predicted a large response rate difference between a variable-ratio and a "linear feedback" variable-interval schedule, contrary to the results of McDowell and Wixted (1986). We concluded that simple IRT reinforcement was probably not adequate as an explanation of schedule effects under aperiodic interval and ratio schedules, although a modification of the Peele et al. model incorporating behaviors that were not measured operants could exhibit schedule sensitivity. This suggested that realistic molecular models of schedule phenomena must involve more than simple IRT reinforcement.