This study tested the ergogenic effects of acute administration of melatonin on exhaustive exercise (tlim) at the anaerobic threshold intensity (iAnT) during periods of lower (L) and higher (H) spontaneous physical activity in swimming rats. Additionally, we evaluated the time of day effect on aerobic exercise tolerance. The periods of L and H were determined gravimetrically. All animals were subjected to an incremental test to determine the iAnT. Melatonin was administered (10 mg.kg(-1), intraperitoneal) and after 30 min, the rats were subjected to tlim during the L (LM) or H (HM) period. Control groups were called LC and HC. The criterion of significance was 5%. Melatonin enhanced tlim by 169% during H (HC=72 min; HM=194 min; P<0.01; ES=1.23) and by 90% during L (LC=31 min vs. LM=59 min; P=0.39; ES=1.18), demonstrating a significant effect on tlim (F=10.35; P<0.01) and a strong effect size (ES). Additionally, tlim was higher during H (F=14.24; P<0.01). Melatonin is a reasonable ergogenic aid, particularly during the wakefulness period, and the exercise tolerance is dependent on the time of day for swimming rats.
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