Eight young children who displayed destructive behavior maintained, at least in part, by negative reinforcement received long-term functional communication training (FCT). During FCT, the children completed a portion of a task and then touched a communication card attached to a microswitch to obtain brief breaks. Prior to and intermittently throughout FCT, extinction probes were conducted within a withdrawal design in which task completion, manding, and destructive behavior were placed on extinction to evaluate the relative persistence of appropriate and destructive behavior over the course of treatment. FCT continued until appropriate behavior persisted and destructive behavior failed to recur at baseline levels during extinction probes. The completion of FCT was followed by four challenges to the persistence of treatment effects conducted within mixed- or multiple-schedule designs: (a) extended extinction sessions (from 5 to 15 min), (b) introduction of a novel task, (c) removal of the microswitch and communication card, and (d) a mixed schedule of reinforcement in which both appropriate and destructive behavior produced reinforcement. The results showed that although FCT often resulted in quick reductions in destructive behavior and increases in appropriate behavior, destructive behavior often recurred during the extinction probes conducted during the initial treatment. When the effects of treatment persisted during the extinction probes, the remaining challenges to treatment effects resulted in only mild to moderate disruptions in behavior. These results are consistent with the quantitative predictions of behavioral momentum theory and may provide an alternative definition of maintenance as constituting behavioral persistence.