Individuals identified originally as constituting a group with low platelet monoamine oxidase (MAO) activity were contacted again two years later to evaluate whether personal and family differences from controls with normal MAO activity might persist. Upon follow-up, the low MAO males, who had originally shown a higher incidence of varied forms of psychopathology, continued to show differences from controls, principally in having more job instability, poorer school performance, and more medical problems; while they did not develop more new mental health problems than did controls, they reported more mental health problems in their families, especially depression, alcoholism and suicide attempts--findings in keeping with other suggestions that low MAO activity might represent a risk factor or familial marker for certain types of psychopathology. In laboratory studies examining substrates for the MAO subtypes, norepinephrine was found to be a less selective substrate in vitro for MAO type A than suggested by previous studies; this was especially so in primates, including man, in keeping with other data indicating that in man compared to rodents, MAO-B plays a larger functional role in the metabolic degradation of some important neurotransmitter substrates such as dopamine and norepinephrine.