The decline in amplitude of the P50 component of the event-related potential to the second of paired clicks has been suggested as a measure of preattentional gating. Two experiments were conducted to assess the effects of attention and a psychological stressor on P50. Experiment 1 included two choice reaction time tasks designed to direct attention selectively to the first or second click in each pair. Results suggest that the N100 component was responsive to attentional manipulations, whereas P50 was not affected. Experiment 2 examined the impact of a brief psychological stressor on the P50 response. Parallel mental arithmetic tasks were administered silently and orally. Self-report and measures of autonomic activity were used to assess the level of stress occurring during the performance of the mental arithmetic tasks. Results indicate that P50 suppression was sensitive to the acute stressor, the oral mental arithmetic task. Implications of these findings for studies of P50 suppression in schizophrenia are discussed.