The N100, P200, N200 and P300 components of the auditory event-related potential were recorded from 10 male subjects at 0900, 1600, and 1830 hours at sea level and again following a rapid ascent to simulated 4300 m altitude. Amplitude and latency of components, ear oximetry, and concurrent performance measures (reaction time and counting errors) were assessed. Amplitude of P300 decreased, while P300 latency and reaction time increased, following ascent to altitude. However, the time course of altitude effects differed for amplitude versus latency. Components N100, P200, N200, and counting errors were unaffected by altitude. The results indicate that central measures of cognitive capacities are differentially sensitive to high altitude. The time course of altitude effects on P300 amplitude versus P300 latency suggests that the two measures reflect different aspects of a response to hypobaric hypoxia exposure.