This study investigated the effects of hypoxia on parallel/preattentional and serial/attentional processes in early vision, and the use of a positive-end-expiratory-pressure (PEEP) to prevent the impairment in performance. Twenty-one subjects were submitted to an 8-h hypoxia exposure in a hypobaric chamber (4500 m, 589 hPa, 22 degrees C), both with and without a 5-cm H2O PEEP. Subjects carried out a visual search task consisting of detecting a target among distractors in normoxia, in acute and in prolonged hypoxia. Conjointly their sensitivity to acute mountain sickness (AMS) was scored through the Lake Louise AMS scoring system. Results showed that prolonged hypoxia slowed serial/attentional processing whereas parallel/preattentional processes were not impaired either by acute or by prolonged hypoxia. PEEP prevented serial/attentional processes from slowing and those effects were more clearly observed in the AMS sensitive subjects with respect to the AMS insensitive subjects. These results suggest that the slowing induced by prolonged hypoxia is specific to an early visual process that pilots the scanning of an attentional spotlight throughout the visual field.