Abstract Evidence exists that the intention to perform certain cognitive tasks activates, unintentionally, competing responses and computations that intrude on the performance of the intended tasks. For the intended task to be performed effectively, such intrusions must be controlled. Two experiments were carried out to test the hypothesis that stress heightens the difficulty of exercising effective control over erroneous competing responses, a possible explanation of decrements in the performance of cognitive tasks under stress. Participants performed four tasks, which contained features that could potentially prime or activate erroneous responses. The results demonstrated that the interference of these features with performance was more pronounced among stressed than among less-stressed participants. The need for a more comprehensive theory of the effects of stress on information processing is discussed.