The study aimed to investigate the relation of both negative mucosal potentials (NMP) obtained from respiratory epithelium, and pain ratings to repetitive stimulation with CO2 (21 subjects). Trains of four stimuli of identical intensity (70% v/v) were applied at a constant interseries interval (approximately 60 s) but different interstimulus intervals (ISI) of 2 s or 6 s. At an ISI of 6 s, ratings decreased while they increased at an interval of 2 s (P < 0.01). This change was accompanied by the buildup of burning pain probably relating to the 'wind-up' of spinal neurons. In contrast, the decrease of NMP amplitudes was stronger the shorter the ISI (P < 0.01). These findings are in line with the view that the NMP reflects activation of epithelial nociceptors (C-fibers and/or A delta-fibers).