Glutamic acid and its analogs are excitotoxins that might contribute to the pathogenesis of Parkinson's disease (PD). We measured four subtypes of glutamate binding sites autoradiographically in 20-microns sections from control and PD midbrains. N-Methyl-D-aspartate (NMDA) binding sites (eight control, eight PD) were very low in control (20 +/- 7 [SEM] fmol/mg protein) and were reduced in the PD pars compacta (2.6 +/- 1.1 fmol/mg protein; p less than 0.02). NMDA binding was also reduced in the red nucleus but not in periaqueductal gray (PAG). We measured alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), metabotropic, and non-NMDA, nonkainate, non-quisqualate (NNKQ) sites in 10 PD and 12 control midbrains. AMPA binding sites were reduced from 175 +/- 20 to 99 +/- 16 (p less than 0.05) fmol/mg protein in PD pars compacta, NNKQ sites from 86 +/- 10 to 50 +/- 12 (p less than 0.05) fmol/mg protein in total nigra, and metabotropic sites (15 +/- 5 fmol/mg protein) were unchanged. AMPA, metabotropic, and NNKQ binding were unchanged in red nucleus and PAG. The very low number of NMDA binding sites suggests that factors other than excitotoxicity mediated via NMDA receptors on nigral cell bodies play roles in the pathogenesis of PD. There may be a generalized loss of NMDA receptors in PD brains. AMPA and NNKQ binding sites appear to be located on dopamine neurons, although the role of NNKQ sites in normal nervous system function and human disease is unknown.