1. Neurochemical, neuropharmacological, and neurophysiological studies suggest that some of the so-called trace amines may have a role in the modulation of neurotransmission. This review examines the possible existence and characterization of brain binding sites for the trace amines. 2. The results of radioligand binding studies carried out so far suggest the existence of tryptamine binding sites that possibly constitute a true functional receptor. This is supported by evidence obtained from the saturation studies, drug-mediated inhibition of binding, and the changes in the number of sites induced by pharmacological and lesion studies. In addition, the existence of a functional tryptamine binding site is supported by the increased neurophysiological responses of tryptamine obtained from the striatum of rat with unilateral substantia nigra lesions. 3. It has been shown that the brain contains saturable binding sites for rho-tyramine that appear to be related to the transport of dopamine into synaptic vesicles. There are, however, some questions with respect to the homogenization technique employed and some inconsistencies with respect to the number of binding sites estimated in neuronal membrane preparations. 4. The existence of rho-octopamine binding sites has been demonstrated in crude membranes obtained from fruitflies but not shown so far in vertebrates. 5. The presence of brain binding sites for beta-phenylethylamine are suggested but they are not so well defined and its physiological implication remains to be elucidated.