Platelet monoamine oxidase (MAO) activity & intellectual level were examined in a sample (n=1129) of 18-year-old boys. Personality traits were determined by personality inventories (ZSSS, EPI & KHPI). Information was also gathered concerning substance abuse. Low MAO subjects were found to be more sensation seeking & to have higher impulsivity & monotony avoidance. Low MAO subjects with lower intellectual levels had more substance abuse & seemed to be the “high risk” group for suicidal behavior.