Four experiments with rats examined Pavlovian incubation, in which responding increases when Pavlovian conditioning is followed by a testing delay. In a within-subjects design, Experiment 1 first showed that when a single food pellet unconditioned stimulus (US) signaled the delivery of three additional pellets, responding after the single US was significantly greater on a delayed, compared to an early, test. Pavlovian incubation was not importantly due to extinction testing (Experiment 2), increased motivation (Experiment 3), or differences in apparatus exposure, or handling cues, during the incubation period (Experiment 4). Implications for theories of conditioning are considered. (Contains 8 figures.)