This chapter discusses some of the main features in investigations utilizing positron emission tomography in studies on neurotransmission and receptor binding. The receptors are usually fixed and may be up- or down-regulated as a function of the mobile agonist. The chapter details the basic features of the synaptic transmission. The neurotransmitter may, after release from the receptor, be deactivated by enzymes to produce metabolites in the cytoplasm or in the membranes. Before selecting an appropriate strategy in the visualization of the turn-over of the neurotransmitter the rate limiting step should be considered. All quantitative analysis of receptor binding in vivo relies on relevant tracer kinetic models. Applications of labelling receptor ligands with gamma emitting radionuclides to be used in single photon spectroscopy have been performed. In order to control problems arising due to metabolism, the possibilty of labelling a ligand in various positions or with different radionuclides might give relevant information on problems due to labelled metabolites.