Emission computer tomography offers a unique opportunity to examine functional receptor processes. With the help of specific exogenous radioligands for the various receptor systems of the brain it is possible to visually depict receptor-neurotransmitter interactions, metabolic consequences and changes in diseased conditions. Although radioligands for application in positron emission tomography (PET) are relatively numerous, those for single photon emission tomography (SPECT) are markedly fewer. However, the general availability of the SPECT method gives the SPECT radioligands a special significance. The known SPECT ligands for brain receptors are introduced and discussed. While for certain receptor systems (eg glutaminergic and opiate) no generally useful substances are available, in the case of other receptors (eg cholinergic, serotonergic, dopaminergic and the benzodiazepine system) certain molecules are undergoing clinical evaluation. Iomazenil, a benzodiazepine antagonist, is on the verge of clinical application. Particularly in the case of partial epilepsy, and also possibly in schizophrenia and Alzheimer's disease, this SPECT radioligand is seen as a clear advance in diagnostic methodology. In the light of Iomazenil studies it has been shown how the reproducible analysis of SPECT images can be improved using the Talairach atlas. Finally, the question of whether and how SPECT methodology could be used to determine receptor affinity constant and receptor density is discussed. It is apparent that, under certain conditions (eg., low unspecific binding, slow kinetic etc.), the determination of affinity constants should be feasible using this approach.