The responses elicited by olfactory stimuli may be modified throughout an organism's life. For example, Wysocki et al. found that regular presentations of 5a-androst-16-en-3-one (androstenone) to anosmic subjects resulted in a graded increase in the perceived intensity of this substance in about half of their subjects (Wysocki et al., 1989). The increased sensitivity they observed appeared to be specific to the exposed odorant and was presumed to occur only in anosmic subjects. Here, we continue our explorations of the individual differences in olfactory capabilities among subjects initially classified by their ability to detect and identify the odor of the diastereoisomeric ketone, cis-4-(4'-t-butylcyclo-hexyl)-4-methyl-2-pentanone (pemenone) which shares with androstenone a pronounced urine-sweaty odor. We asked if regular pemenone exposure enhances the sensitivity of human subjects to pemenone, androstenone, isovaleric acid, or phenylethyl alcohol and, if shifts in threshold occurred, were they specific to particular odorants and classes of subject? Detection thresholds for the four substances were determined before and after 7–8 weeks of regular, biweekly, exposure to pemenone (n = 18 subjects) or a control substance (22 subjects). Significant decreases in threshold were seen in the experimental group, relative to the control group, for androstenone, but not for the other compounds evaluated. Neither gender nor initial pemenone osmicity significantly affected the frequency of subjects with enhanced sensitivity. These findings show that a subject's sensitivity to one odorant can be enhanced by exposure to another, chemically distinct compound. Enhancement was not limited to anosmic individuals, but was also observed in some initially osmic and allosmic subjects. Chem. Senses 20: 413–419, 1995.