Regional cerebral blood flow (CBF) measurements were conducted under resting conditions and after twenty-four hours of caffeine abstinence in groups of high and low caffeine users. CBF was also measured thirty minutes after the oral administration of 250 rag. of caffeine in the high caffeine users and after a placebo in the low caffeine users. Caffeine administration was associated with significant CBF reduction and caffeine abstinence with significant frontal flow increases in the high caffeine group. The low caffeine group showed no differences between the resting, post-placebo and post-caffeine abstinence CBF values.