Previous studies have established the presence of a middle-latency auditory-evoked potential that is characterized by a) sleep-atote dependence, b) low foflowing frequency (i.a., rapid habituation to repetitive stimulation), and c) blockade by the cholinerglc antagonist, scopolamine. A vertex-recorded evoked potential having these characteristics was described in humans at a 5080 ms latency (termed the P1 or P50 potential and in the cat at a 20–25 ms latency (termed wave A). These studies were undertaken to determine if a dick stimulus-evoked potential having the same characteristics was present in the intact rat Vertex and auditory cortex recordings in intact rats studied in a sound-attenuating chamber and exposed to irve-field dick stimuli showed a) the presence of a vertex recorded potential at a 11–15 ms latency, termed P13, and of an auditory cortex recorded potential at a 7–11 ms latency, termed Pa; b) the P13 was present during waking and paradoxical sleep but absent t slow-wave sleep, while Pa was present in all sleep-wake states; c) the P13 habituated markedly at stimulation rates above 1 Hz while Pa did not; and d) the P13 was blocked by low doses of scopolamine while Pa was riot These studies demonstrate the presence of a P1-like potential in the rat at a 13 ± 2 ms latency.