Forty Army volunteers were given a synthetic tetrahydrocannabinol (THC) compound similar in structure and physiological activity to the active component of marihuana. Significant relationships were found between the personalities of these volunteers as measured by the Minnesota Multiphasic Personality Inventory (MMPI) and the Army General Intelligence Test (GT), and performance on objective tests following administration of this compound. Several MMPI scales and the GT score appeared to be as strongly correlated with performance as was the dose level. MMPI and GT test interpretation of subjects resistant to the performance impairment caused by this compound showed them to be more intelligent, adventurous, and more hostile and aggressive than sensitive subjects. A possible explanation for continued use of marihuana by certain personality types is offered.