Past studies on relationships between drug use and personality are open to criticism on theoretical and methodological grounds. Properly conducted research may show relationships that have been previously undiscovered. The present studies examined 45 carefully selected, noninstitutionalized men in four matched groups of heavy, chronic drug users and a nonuser comparison group. A variety of instruments, all of which presumably measure the propensity to seek novel and varied sensations, were administered. When used to evaluate hypotheses derived from Optimal Level of Arousal (OLA) theory, data from this study showed an association between the trait measured and drug(s) of choice among men committed to the heavy, chronic use of particular substances. However, the association was not that anticipated by OLA theory. Instead, it appeared that, among heavy, chronic users, drug preference and sensation seeking are related to extraversion and introversion. The findings were consistent with a theory of psychological individuation that was developed from thorough clinical assessment of every individual in the research program.