Amplitude of the contingent negative variation component of the EEG was assessed in 18 subjects exposed to several odor conditions. Three primary odors were used as well as a mixture of the primary odors. Subjects were led to believe that the odor mixture was actually three different odors which were low concentrations of each of the primary odors. CNV amplitude changed as a function of subjects' expectations about this mixed odor rather than the direct physiological effects of the odor stimulus. These results suggest that EEG changes to odors may be the result of cognitive mediation rather than direct CNS changes induced by the odors.