Equine pituitary pars intermedia dysfunction (PPID) is a spontaneously occurring progressive disease affecting aged horses and ponies. The pathogenesis of PPID is poorly understood, but the available evidence supports a loss of dopaminergic inhibition of the melanotropes of the pars intermedia. Horses with PPID have increased plasma concentrations of pars intermedia pro-opiomelanocortin-derived peptides that decrease in response to dopamine or dopamine agonist administration. Dopamine and dopamine metabolite concentrations are decreased in the pars intermedia of affected horses compared to age-matched control horses. Horses with disease that are treated with the dopamine agonist pergolide show improvement in clinical signs and normalisation of diagnostic test results. In the present study, immunohistochemical evaluation of pituitary and hypothalamic tissue demonstrated reduced tyrosine hydroxylase immunoreactivity in affected horses compared to age-matched and young controls, supporting the role of dopaminergic neurodegeneration in PPID. In addition, immunohistochemical evaluation revealed an increase in the oxidative stress marker, 3-nitrotyrosine and in nerve terminal protein, α-synuclein that colocalised in the pars intermedia of horses with disease. These findings suggest a role for nitration of overexpressed α-synuclein in the pathogenesis of neurodegeneration in PPID.