Behavioral changes after administration of the neurohypophyseal hormones vasopressin and oxytocin can be observed in animal and man. Several groups attempted to specify these changes in terms of memory or attention processing enhancement for vasopressin and amnesic properties for oxytocin. These interpretations, however, were targets for recent criticism. In a double-blind between-subject comparison with male volunteers receiving arginine-vasopressin (AVP), oxytocin or placebo intranasally prior to the experimental session, we tried to develop an alternative hypothesis on the basis of behavioral and EEG measures. At the beginning of the session subjects had to learn a list of 25 unrelated nouns within five trials. Recall was assessed 1 h later. Neither learning nor long-term recall were affected by peptide treatments. In a second vigilance task subjects had to covertly count eight series of tone pips. Averaged auditory evoked potentials to these tones showed the expected habituation during the course of the task within all three groups. Vasopressin-treated subjects, however, displayed significantly higher amplitudes of the vertex potential as compared to the other treatment groups. AVP effects were most prominent with the longest interstimulus interval. No influences on heart rate or blood pressure were found. Results indicate that vasopressin induces an enhancement of stimulus-related phasic cortical arousal, and that in this respect oxytocin has no effect.