Biogenic amines play key roles in neurotransmission, metabolism, and in control of various physiological processes.  Ring-halogenated analogs have proved to be powerful tools for the study of the mechanisms of transport, storage, release, and modes of action of these amines since they simulate the geometries of the natural compounds so well.  By virtue of its very small size and high electronegativity, fluorine should be a very favorable replacement for hydrogen in these analogs.  Some years ago, we developed novel methods for the synthesis of such analogs and have prepared a wide variety of biogenic amines with fluorine at various ring positions.  The biological properties and usefulness of these analogs have proved unexpectedly rewarding and continue to find application in a multitude of studies.  Perhaps the most significant finding, to date, is that 2-fluoronorepinephrine is a pure beta-adrenergic agonist, while the 6-fluoro isomer is a pure alpha-adrenergic agonist.  Various explanations for the role of fluorine in creating such specificity have been considered and discarded.  We now propose that a critical ion-dipole interaction exists between a positively-charged site on the receptor and an electron-rich carbon atom of the benzene ring.  Inductive withdrawal by fluorine would weaken this interaction and produce the observed specificities.  Results from pharmakokinetic studies of ring-fluorinated catechol amino acids demonstrate their potential as biological tracers, including use as 18F-PETT scanning agents.  In certain of these analogs, fluorine serves not only as the radioactive tracer (18F), but also to retard an undesirable metabolic process (methylation by catechol-0-methyl transferase).  Fluoride release during anodic oxidation of fluorinated amines and amino acids has prompted a study of these compounds as prodrugs for melanoma chemotherapy.