Rarely do we encounter a single odorant in our environment. Perception of odours, therefore, usually depends on the reception and neural processing of many components. However, little is known about how and where odour mixtures are processed. Evidence is presented here that suppression of one odour by another, a common result of mixing odours, is primarily a peripheral event. Having demonstrated with human subjects that perception of one or both odorants in two-component mixtures is dependent on the polarity and perceived intensity of the odorants, the same mixtures were presented to rats that had been injected with a metabolic marker, [3H]2-deoxyglucose (2-DG). By measuring the metabolic activity in the glomeruli of the rat olfactory bulb, where the axons of the receptor cells terminate, it was found that in a mixture where humans had perceived only one odour, there is a dramatic reduction in metabolic activity of glomeruli specific to the suppressed odour. In mixtures where both odorants were perceived, metabolic activity characteristic of both components was observed. These findings indicate that similar mechanisms underlie the perception of odour mixtures in the two species. Since metabolic activity revealed by 2-DG in glomeruli occurs predominantly in presynaptic receptor axons, the reduced activity seen after stimulation with odour mixtures indicates that a mechanism for mixture suppression begins at the receptor cells. Therefore, the ability of one odorant to suppress another in a mixture is probably determined by their relative chemical polarities, which effects access to and competition for membrane receptor sites in the olfactory epithelium.