arrow]US associations also survived The memories of the unconditioned stimulus (US) and its absence (No US), symbolized as S[superscript R] and S[superscript N], respectively, may be retrieved on US or No US trials giving rise to four types of associations, S[superscript R][right arrow]US, S[superscript R][right arrow]No US, S[superscript N][right arrow]US, and S[superscript N][right arrow]No US. Here, following acquisition under partial reward (PRF), rats were shifted either to different schedules of PRF (Experiment 1) or extinction (Experiments 1 and 2). Inhibitory S[superscript R][right arrow]No US associations formed in acquisition survived extinction and shifts to one, but not another type of PRF schedule (Experiments 1 and 2). Excitatory S[superscript R][right extinction (Experiment 2). These findings, as well as the acquisition findings of Experiment 2, are consistent with the sequential model but not with the only other two theories said to be able to explain PRF findings, the frustration hypothesis of Amsel and the attention hypothesis of Mackintosh [see Haselgrove, M., Aydin, A., & Pearce, J. M. (2004). A partial reinforcement extinction effect despite equal rates of reinforcement during Pavlovian conditioning. "Journal of Experimental Psychology: Animal Behavior Processes," 30, 240-250]. Also, the reacquisition findings obtained here are inconsistent with two views applied to several learning phenomena, the rule-learning view and the position-item view. (Contains 2 figures.)