Selection of an adequate placebo is a major problem in clinical trials of Euminz((R)) (10% peppermint oil/ ethanol) which is used topically for the treatment of tension-type headache. This randomized, controlled, double-blind, cross-over study was performed to investigate whether there are qualitative differences between 10%, 1%, 0.5%, 0.1%, and 0% peppermint oil, Forty-one healthy subjects participated (age range 21-28 years); they rated both intensity, and hedonic tone of the stimuli. Verbal descriptions were combined to multiple response sets (MRS). In addition, the trigeminal impact of odorants was determined. Intensity ratings and MRS 'menthol like' and 'alcohol/solvent' changed with stimulus concentration. However, intensity had no significant effect on hedonics, trigeminal impact, or the number of descriptive items used, When MRS 'menthol like' and 'alcohol/solvent' were analysed after being weighted with intensity ratings, changes in relation to stimulus concentration were lost. Thus, the differences between the five concentrations of peppermint oil were-to their largest part-due to changes in stimulus intensity. Considering the large day-to-day variability of olfactory sensitivity the present data support the hypothesis that the odour quality of 10% peppermint oil cannot be discriminated from the odour of 0.1%, 0.5%, or 1% peppermint oil when tested on separate days. Copyright (C) 2001 John Wiley & Sons, Ltd.