PURPOSE:This study explored conflicting findings in the literature on the influence of perceived cognitive fatigue on physical performance by testing the effect of the Stroop task (high cognitive load) on an intermittent running test (beep test). METHODS:In a within-subjects repeated-measures experiment, 13 active athletes performed the beep test on two occasions, in a randomized, counterbalanced order. In each session, a preceding cognitive task was completed for 30 min, with the incongruent Stroop task in the high load condition, and the congruent Stroop task in the low load condition. Perceived cognitive fatigue was measured before testing (baseline) and at 10, 20, and 30 min of the cognitive load manipulation. Perceived effort on the cognitive task and general motivation for the physical task (beep test) were measured before the beep test, and motivation-related perception of the beep test and ratings of perceived exertion were measured after completion of the test. Heart rate and beep test performance (completion stage and time) were also recorded. RESULTS:The incongruent Stroop task was perceived as more fatiguing and effortful. Participants also withdrew from the beep test significantly earlier in the high load condition (M = 8:48 min, SD = 2:32 min) compared to the low load condition (M = 9:20 min, SD = 2:28 min), F (1,11) = 21.76, P < 0.01, ŋ = 0.67. There were no differences in heart rate or general motivation between the two conditions. CONCLUSIONS:Whereas previous research shows that active athletes can maintain performance on the beep test after 10 min of the incongruent Stroop task, this study shows that performance is impaired after 30 min. Variables in need of exploration in future investigations include experience with both the physical and cognitive task.