Caffeine is thought to have stimulant-like behavioral effects on mood and performance. However few behavioral studies have examined this substance's acute effects when administered in a range of doses that include the low doses typically found in foods and over-the-counter drugs. We therefore gave single doses of caffeine (32, 64, 128 and 256 mg) to 20 healthy male subjects and assessed various aspects of performance and self-reported mood states, as well as plasma caffeine concentration. As little as 32 mg (which elevated plasma caffeine concentration to less than 1 microgram/ml), typical of the dose found in a single serving of a cola beverage, and less than that found in a single cup of coffee or a single dose of over-the-counter drugs, significantly improved auditory vigilance and visual reaction time. All other caffeine doses administered also significantly improved performance on these tests. No adverse behavioral effects, such as increased anxiety or impaired motor performance, were noted even at the highest dose administered.