Localization of the mRNAs for fractalkine, a CX₃C chemokine, and for its receptor CX₃CR1 was investigated in the rat brain. In situ hybridization study revealed that fractalkine mRNA was dominantly expressed in neuronal cells particularly in the olfactory bulb, cerebral cortex, hippocampus, caudate putamen and nucleus accumbens. In vitro study using enriched neuronal or glial culture supported the dominant expression of fractalkine mRNA in neurons. On the other hand, CX₃CR1 mRNA was dominantly expressed in glial cells throughout the whole brain. The in vitro study suggested the cells expressing CX₃CR1 mRNA are microglia, not astrocytes or neurons. Fractalkine appears to function as a signal molecule from neuron to microglia.