These experiments were motivated by the idea that many types of nondeclarative memory are by-products arising from the plasticity that is inherent in much of the nervous system. We hypothesized that two types of repetition priming, word-stem completion (WSC) priming and perceptual identification (PI) priming, rely on different mechanisms because the WSC task and the PI task engage different cognitive and brain processes. We tested this hypothesis by manipulating word familiarity. The results, impaired WSC priming but intact PI priming with unfamiliar words, indicate that WSC priming relies primarily on a modification mechanism, whereas PI priming relies primarily on an acquisition mechanism. Our conclusions are consistent with component processes theories of nondeclarative memory.