Infants trained on FI schedules are able to adjust their post-reinforcement pause, but many sessions at a given parameter value are necessary before the FI cumulative record appears stable. We analysed transitions from one FI value to another from our earlier experiments. Infants were exposed to FI schedules from 10 to 80 s. Under these schedules, they showed long post-reinforcement pauses with most intervals involving the emission of a single, reinforced response. The passage from one FI value to another was unpredictable for infants. We observed rapid timing effects during transitions (passage from one FI value to another, from FI 10 s to FI 20 s, etc.). Wait times increased during the first session of the new FI value, and accurate adjustments of wait times quickly appeared. Dynamic effects observed were similar to those obtained from animals in experiments on rapid-timing effects.