Brain catecholamine and serotonin neurons are intimately involved in a number of physiological functions such as cardiovascular regulation, neuroendocrine secretion, regulation of behavior, mood and appetite control. The synthesis and release of these neurotransmitters appear to be remarkably correlated with the subsequent physiological changes observed. While there are numerous pharmacological approaches that affect the neurotransmitters involved (i.e., the catecholamines dopamine, noradrenaline and adrenaline as well as the indoleamine serotonin), one particularly attractive (while specific) approach involves the administration of a precursor amino acid. It is thus a prerequisite for the physiological and possibly therapeutic properties of the precursor amino acids l-tyrosine and l-tryptophan that they affect synthesis and release of their transmitter product. We will describe in detail the nature of the different neurochemical mechanisms guiding transmitter synthesis and their dependent physiological responses, in particular appetite control and neuroendocrine regulation.