Publisher Summary This chapter discusses genetic differences in mechanisms involving neuroregulators. The potential role of genetic factors becomes of great importance in terms of a number of theories of the illness. The possibility of genetic variation in catecholamine synthesizing activity is suggested by several lines of evidence. In animals, strain and subline differences have been reported in the amounts of biogenic amines in brain regions of mice and rats and in the utilization and uptake of cardiac norepinephrine in mice. In humans, a variety of forms of pheochromocytoma have been shown to be associated with familial factors. There are known genetic variations in adrenocortical and adrenomedullary structure. Genetic variations in steroid hormones produced by the adrenal cortex and in thyroid hormone have been associated with behavioral changes, and further e search for genetic variation in the enzymes involved in catecholamine biosynthesis would be fruitful. The use of inbred mouse strains is particularly advantageous in studies of genetic variation and provides a powerful tool for subsequent behavioral and genetic analysis.