[3H]-spiperone is commonly used to label dopamine receptors in vitro in brain tissue. However, spiperone also interacts with brain 5-hydroxytryptamine and noradrenaline receptors. In vivo, [3H]-spiperone has been used for identifying dopamine receptors in both animals and man but the nature of the sites identified is unknown. The in vivo administration of [3H]-spiperone to rats leads to a selective accumulation of radioactivity in the olfactory lobes, tuberculum olfactorium, nucleus accumbens, striatum, substantia nigra, hippocampus, frontal cortex and hypothalamus, when compared to the cerebellum. In vivo drug displacement studies suggest that the binding of [3H]-spiperone in these areas may be to dopamine, 5-HT or noradrenaline receptors. [3H]-spiperone in vivo mainly labels dopamine receptors in striatum, tuberculum olfactorium, hypothalamus, substantia nigra and olfactory lobes. However, in the frontal cortex and nucleus accumbens specific binding involves not only dopamine receptors but also 5-HT and/or noradrenaline receptors. Interpretation of in vivo studies in man using radioactive spiperone and its derivatives must take into account the fact that this ligand only labels dopamine receptors in some brain areas.