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Physiological studies have shown that synaptically released zinc plays an important role in neural signaling by modulating a number of excitatory and inhibitory neurotransmitter receptors and intracellular signaling proteins. In order to localize neurons having a zincergic phenotype, Slomianka et al. [Neuroscience 38 (1990) 843] developed a labeling technique, based on the systemic administration of sodium selenite, that results in the retrograde transport of zinc-selenide crystals from axonal boutons to the cell bodies of origin. A major problem associated with this method is that the zincergic neurons are obscured by high levels of staining within synaptic boutons. In the present study, we describe a modification of the procedure for retrograde labeling of zincergic neurons, that uses a preincubation step with H2O2, which eliminates labeling of axon terminals while leaving the staining of cell bodies intact. Using this method we reveal that zincergic neurons comprise a large proportion of neurons in the murine forebrain, underscoring their contribution to network properties therein. [Copyright &y& Elsevier]