There is a shortage of evidence regarding exactly how mental imagery works to enhance performance. This study sought to determine whether it is the imagining of the task that is crucial or simply the positive aspects of a mental image. In the first experiment, 52 male students used task relevant imagery, task irrelevant imagery, or a distraction control procedure before performing an analogue task. Those in the task relevant condition showed significantly greater improvements over baseline. The second experiment involved 142 male students and included assessment of mood state following psyching up. Again the task relevant group showed significantly greater improvements, which were not related to mood states. These findings suggest that the specific content of mental imagery is crucial in determining its effect on performance. The effect does not appear to depend on alterations of mood state and may operate through cognitive preparation.