Oxidative stress is thought to be involved in the mechanism of nerve cell death in Parkinson’s disease (PD). Among several toxic oxidative species, nitric oxide (NO) has been proposed as a key element on the basis of the increased density of glial cells expressing inducible nitric oxide synthase (iNOS) in the substantia nigra (SN) of patients with PD. However, the mechanism of iNOS induction in the CNS is poorly understood, especially under pathological conditions. Because cytokines and FcεRII/CD23 antigen have been implicated in the induction of iNOS in the immune system, we investigated their role in glial cells in vitro and in the SN of patients with PD and matched control subjects. We show that, in vitro, interferon-γ (IFN-γ) together with interleukin-1β (Il-1β) and tumor necrosis factor-α (TNF-α) can induce the expression of CD23 in glial cells. Ligation of CD23 with specific antibodies resulted in the induction of iNOS and the subsequent release of NO. The activation of CD23 also led to an upregulation of TNF-α production, which was dependent on NO release. In the SN of PD patients, a significant increase in the density of glial cells expressing TNF-α, Il-1β, and IFN-γ was observed. Furthermore, although CD23 was not detectable in the SN of control subjects, it was found in both astroglial and microglial cells in parkinsonian patients. Altogether, these data demonstrate the existence of a cytokine/CD23-dependent activation pathway of iNOS and of proinflammatory mediators in glial cells and their involvement in the pathophysiology of PD.