The present study examines the psychological characteristics and self-reported responses to xenobiotic agents such as tobacco smoke and pesticide of normal young adults with personality traits similar to those claimed for Parkinsonian patients. Previous research, though controversial, has suggested that persons with idiopathic Parkinson's disease (PD) have premorbid personality traits that may include shyness and repressive defensiveness. Other epidemiological evidence indicates that PD patients may have premorbidly increased prevalence of anxiety, affective, and/or somatoform disorders; decreased rates of smoking and alcohol consumption; and elevated exposure to herbicides or pesticides. A total of 783 college students enrolled in an introductory psychology course completed the Cheek-Buss Scale (shyness), the Marlowe-Crowne Social Desirability Scale (defensiveness), Symptom Checklist 90 (revised), the Mastery Scale, a health history checklist, and rating scales for frequency of illness from alcohol and 10 common environmental chemicals. Subjects were divided into four groups on the basis of above-versus below-median scores on the Cheek-Buss and Marlowe-Crowne scales (persons high in shyness and defensiveness, those high only in shyness, those high only in defensiveness, and those low in both shyness and defensiveness). The group high in shyness but low in defensiveness had the highest, whereas the group low in shyness but high in defensiveness had the lowest, total scores on the SCL-90-R; the two shyest groups were lowest in sense of mastery. Similar to PD, the group high in both shyness and defensiveness overall reported the least number of smokers (10% vs. 19% in those high only in shyness, 17% in those high only in defensiveness, and 28% in those low in both traits,p< .001); differences within women largely accounted for this finding. Subjects higher in shyness and/or defensiveness rated themselves higher in frequency of illness from a small amount of alcohol than did those who were low in both shyness and defensiveness. The group who was high in both shyness and defensiveness tended to report the highest frequency of illness from pesticide as well as other xenobiotic odors (e.g., newsprint). Taken together with previous research, the findings suggest that certain young adults high in shyness, and especially those also high in defensiveness, may be among the subset of the population at increased risk for PD later in life.