Day-night differences of trigeminal chemosensitivity were investigated in 18 healthy volunteers employing both pain-related cortical potentials and pain ratings in response to stimulation of the nasal mucosa with CO2. Day-night differences were found with N1P2 amplitudes, P2 latencies and pain ratings. It is concluded that the time of the day must not be ignored when human chemosensitivity is investigated at suprathreshold levels.