The time course of the effects of caffeine (200 mg), diazepam (8 mg) and placebo on CNV magnitude, reaction time and state of mood was investigated in normal volunteers using a double-foreperiod paradigm. CNV changes were quantified by means of a linear robust fitting technique. In general, a reduction of amplitude was found for diazepam and an enhancement followed by a suppression in the caffeine condition. Emotional lability of the subjects, as measured by the Freiburg Personality Inventory was shown to strongly influence the nature and time course of these drug effects. Differential drug responses are discussed in terms of the attention arousal model.