Inhaled vapours may stimulate both olfactory receptor cells and endings of the trigeminal nerve. The trigeminal nerve often contributes a pungent, irritating attribute to odours. Many odorants exhibit some pungency when dilute and most become pungent when concentrated. Because the trigeminal system commonly shares the chemosensory burden with olfaction, it is relevant to ask whether these anatomically distinct systems interact. Two obscure psychophysical observations argue for an inhibitory influence of trigeminal over olfactory activity. Katz and Talbert observed that a vapour with both odour and pungency might lose odour at high concentrations, irritation masking odour. The nineteenth century philosopher Alexander Bain, nothing that concentrated carbon dioxide (carbonic acid) evokes pungency, remarked "if a current of carbonic acid accompanies an odour, the effect (odour) is arrested" (ref. 8). We have taken up Bain's forgotten observation and used carbon dioxide to endow otherwise benign concentrations of odorant with varying degrees of pungency. The experiments reported here reveal a strong mutual interaction between pungency and odour, occurring without attenuation even when irritant enters one nostril and odorant the other.