This experiment addressed the issue of whether the changes in semantic memory performance associated with Alzheimer's disease (AD) could be distinguished from a generalized cognitive slowing. Young adults, healthy older adults, and AD patients performed 3 different reaction time (RT) tasks involving yes-no responses to visually presented letter strings. Task complexity analyses indicated that performance in the semantic task (lexical decision) was consistent with a generalized slowing of cognitive function that was greater in magnitude for AD than for normal aging. Signal detection analyses of the lexical decision data demonstrated AD-related changes in word-nonword discrimination, response bias, and the relation between discrimination and RT. The general cognitive slowing associated with AD was accompanied by additional changes specific to the performance of this semantic memory task.