Auditory evoked potentials were recorded in a choice reaction-time task. This test paradigm elicited the attention-related P300 component and was used to study cognitive processing. Compared with age-matched controls, 17 patients with dementia of the Alzheimer type were shown to have significantly longer reaction times as well as delayed latencies of several components of the auditory evoked potentials. The fractional increase in the reaction times was much greater than that of the P300 peak latency. The latter is commonly accepted as an index of stimulus-evaluation time. These findings suggest a delay in both response selection and stimulus evaluation.