The possible involvement of 1-methyl-4-phenyl-pyridinium ion (MPP+) in the toxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) prompted us to search for and characterize [3H]MPP+ binding sites in the mouse. Our data show that [3H]MPP+ binds saturably and with high affinity to mouse brain membranes. Scatchard analysis resulted in one straight line. The apparent KD was 15 +/- 1 nM and the Bmax 245 +/- 30 fmol/mg protein. The distribution of [3H]MPP+ binding sites shows a regional variation: the hypothalamus having highest binding and the cerebellum the lowest. Several compounds failed to inhibit [3H]MPP+ binding whereas only analogues of MPP+, MPTP and paraquat were able to antagonize this binding to brain. Specific binding with analogous characteristics also occurs in peripheral tissues. Considering the postulated role of MPP+ in MPTP neurotoxicity, further studies on [3H]MPP+ binding sites might be relevant to elucidate the mechanisms of this toxicity.