The cannabinoid system is known to be important in neuronal regulation, but is also capable of modulating immune function. Although the CNS resident microglial cells have been shown to express the CB₂ subtype of cannabinoid receptor during non‐immune‐mediated pathological conditions, little is known about the expression of the cannabinoid system during immune‐mediated CNS pathology. To examine this question, we measured CB₂ receptor mRNA expression in the CNS of mice with experimental autoimmune encephalomyelitis (EAE) and, by real‐time PCR, found a 100‐fold increase in CB₂ receptor mRNA expression during EAE onset. We next determined whether microglial cells specifically express the CB₂ receptor during EAE, and found that activated microglial cells expressed 10‐fold more CB₂ receptor than microglia in the resting state. To determine the signals required for the up‐regulation of the CB₂ receptor, we cultured microglial cells with combinations of γ‐interferon (IFN‐γ) and granulocyte) macrophage‐colony stimulating factor (GM‐CSF), which both promote microglial cell activation and are expressed in the CNS during EAE, and found that they synergized, resulting in an eight to 10‐fold increase in the CB₂ receptor. We found no difference in the amount of the CB₂ receptor ligand, 2‐arachidonylglycerol (2‐AG), in the spinal cord during EAE. These data demonstrate that microglial cell activation is accompanied by CB₂ receptor up‐regulation, suggesting that this receptor plays an important role in microglial cell function in the CNS during autoimmune‐induced inflammation.