To elucidate the effect of γ-aminobutyric acid (GABA) on both psychological and physical fatigue and on the performance advances for task solving, we assigned an arithmetic task for the Uchida-Kraepelin Psychodiagnostic Test (UKT) to 30 healthy Japanese subjects, 9 of whom were diagnosed as having chronic fatigue. The subjects were administered 250 mL of a test beverage containing GABA at the dose of 0, 25, and 50 mg before assigning task for the UKT. Psychological fatigue assessed by the Visual Analogue Scale (VAS) was significantly lower in the group administrated the beverage containing 50 mg GABA than in the control group (p<0.05). The results of the Profile of Mood States (POMS) also indicated that psychological fatigue was significantly reduced in the 50-mg-GABA group. The salivary secretion levels of chromogranin A and cortisol—markers of physical fatigue—in both 25-mg and 50-mg-GABA groups were significantly lower than those in the control group. The 50-mg-GABA group also showed higher score on UKT by solving the arithmetic task more accurately than the control group (p<0.01). The results suggest that intake of GABA-containing beverages, especially those containing 50 mg of GABA, may help reduce both psychological and physical fatigue and improve task-solving ability.