The goal of my research is to gain a better understanding of human psychopathology through the study of biochemical and behavioral variation in lower animals. Presently this objective is being pursued through a study of normal variation in biogenic amine metabolism. Monoamine oxidase (MAO) activity has been examined in brain and liver of selected inbred strains of mice. Variability in MAO activity was demonstrated among the strains, but the nature of the inheritance of these differences has not been studied. The rank ordering of the strain means suggests a high correspondence between liver and brain MAO activity as well as a high positive association between the activities of the A and B forms of MAO, as measured with the substrates phenylethylamine, serotonin and tyramine. Environmental variables and genotype-environment interactions have also been found to be of importance in controlling activity levels of MAO.