The reinforcing event is present as a stimulus in the situation in which a response is acquired and is absent from the extinction situation. When extinction conditions are made more similar to those of acquisition by introducing the reinforcer unrelated to behaviour (‘free’ reinforcement), the extinguished response is restored. Experiments with students, pigeons, and rata are described; and an analysis of the stimulus effects of reinforcement is used to account for ‘disinhibition’, ‘spontaneous recovery’, and speedy reconditioning after extinction.