Four experiments are reported which demonstrate the ability of an unconditioned stimulus (UCS) presentation following extinction to partially reinstate the conditioned response. These experiments are interpreted in terms of the strengthening of an extinction-reduced UCS representation. The first two experiments address alternative interpretations in terms of sensitization, reinstating the stimulus conditions of acquisition, conditioning of background cues, and stimulus generalization. Experiment 3 suggests that reinstatement is possible with a UCS qualitatively different from that used in conditioning. Experiment 4 explores an alternative extinction procedure which especially preserves the conditioned stimulus-unconditioned stimulus association while encouraging modification of the UCS representation. The results are discussed both in terms of related empirical phenomena, such as spontaneous recovery and sensory preconditioning, and in relation to the general role of the UCS representation in conditioning.