The present study aims to investigate the effect of mentally demanding cognitive tasks on rowing performance in prepubertal athletes. Design: Randomized, counterbalanced and crossover.Method: Seventeen rowers, aged between 10 and 12 years, completed three separate testing sessions during which they performed three different cognitive tasks before completing a 1500 m time trial on the rowing ergometer. In the two experimental conditions, one hour of a standard cognitive task (Stroop task) and an arithmetic school test were used to elicit mental effort; in the control condition a time-matched, not demanding activity was carried out (painting). Subjective workload and mood were measured before and after the treatments, and the motivation was recorded before the time-trial. During the time trial, time, power, speed, cadence, heart rate and rate of perceived exertion were assessed.Results: The Stroop task and the arithmetic test were rated more mentally demanding (P < 0.001), effortful (P < 0.001) and frustrating (P = 0.001) than the control task, but the items fatigue (P = 0.437, P = 0.197) and vigour (P = 0.143, P = 1.000) after the cognitive tasks were not significantly different from the control. The performance of the time trial did not differ between conditions (time: P = 0.521; power: P = 0.208; speed: P = 0.341); physiological (P = 0.556) and perceptual (P = 0.864) measures recorded during the physical task were not affected by the conditions. Accordingly, pacing profiles (P = 0.312) and cadence (P = 0.062) did not differ between the conditions.Conclusions: Mentally demanding activities did not affect the subsequent physical performance in prepubertal athletes.