Coenzyme Q10 (CoQ10) is widely consumed as a dietary supplement to enhance bioenergetic capacity and to ameliorate the debilitative effects of the aging process or certain pathological conditions. Our main purpose in this study was to determine whether CoQ10 intake does indeed attenuate the age-associated losses in motor, sensory, and cognitive functions or decrease the rate of mortality in mice. Mice were fed a control nonpurified diet or that diet containing 0.68 mg/g (low dosage) or 2.6 mg/g (high dosage) CoQ10, starting at 4 mo of age, and were tested for sensory, motor, and cognitive function at 7, 15, and 25 mo of age. Amounts of the ubiquinols CoQ9H2 and CoQ10H2 measured in a parallel study were augmented in the cerebral cortex but not in any other region of the brain. Intake of the low-CoQ10 diet did not affect age-associated decrements in muscle strength, balance, coordinated running, or learning/memory, whereas intake at the higher amount increased spontaneous activity, worsened the age-related losses in acuity to auditory and shock stimuli, and impaired the spatial learning/memory of old mice. The CoQ10 diets did not affect survivorship of mice through 25 mo of age. Our results suggest that prolonged intake of CoQ10 in low amounts has no discernable impact on cognitive and motor functions whereas intake at higher amounts exacerbates cognitive and sensory impairments encountered in old mice. These findings do not support the notion that CoQ10 is a fitness-enhancing or an "antiaging" substance under normal physiological conditions.