Performance on a complex, four-choice, psychomotor task was measured both under self-paced conditions and under various levels of time constraint. Superior performance was produced by moderate reduction, relative to a self-paced baseline, of the time available for task completion, but this effect was exhibited only by subjects high in neuroticism. No contribution of introversion-extraversion could be demonstrated. Although cardiac acceleration was evident under some levels of constraint, the results cannot wholly be attributed to changes in arousal.