As shown in animal studies, 5HT1B/1D agonists can inhibit activity in the trigeminal nucleus caudalis, which may be advantageous for their antimigraine effect. To demonstrate a possible central nervous system (CNS) action of these compounds in man we studied their effect on the intensity dependence of the cortical auditory evoked potentials (IDAPs), thought to be inversely related to central serotonergic transmission. An amplitude/stimulus intensity function (ASF) slope was computed in healthy volunteers and migraine patients between attacks before and 2 h after oral 311C90 (zolmitriptan “Zomig”) 10 mg (n=14), 311C90 5 mg (n=7), sumatriptan 100 mg (n=14), dexfenfluramine 15 mg (n=4), lorazepam 1.25 mg (n=4) and placebo (n=14). 311C90 10 mg and, to a lesser degree, 5 mg significantly increased the mean ASF slope (p=0.007 and 0.05 vs placebo). There was a significant positive correlation between plasma levels of 311C90 and ASF slope changes. Sumatriptan and lorazepam had little effect, but dexfenfluramine produced a significant ASF slope decrease. 311C90 is able to modify a CNS activity that is modulated by serotonin, i.e. the IDAP. This effect is probably the consequence of its super or lipophilicity compared to sumatriptan and of activation of prejunctional 5HT1B/1D autoreceptors, which lowers central serotonin release and thus the preactivation level of sensory cortices.