The compound 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) causes selective destruction of nigrostriatal dopaminergic neurons in primates, giving rise to a condition resembling Parkinson's disease. The toxicity of MPTP is believed to be due to its metabolite 1-methyl-4-phenylpyridinium (MPP+). MPP+ is an inhibitor of mitochondrial respiration at the NADH-ubiquinone oxidoreductase site and this, together with its selective transport into dopaminergic nerve terminals, accounts for its neurotoxicity. In this paper an electrode selective for MPP+ was developed and used to measure the rate of uptake and the steady-state accumulation of MPP+ in rat liver mitochondria. The initial rates of MPP+ uptake were not saturable, confirming previous work that the transport of MPP+ is not carrier-mediated. The membrane potential of mitochondria respiring on succinate was decreased by MPP+ and the steady-state accumulation ratio of MPP+ did not come to equilibrium with the mitochondrial transmembrane potential gradient (delta psi). The effect of the cation exchanger tetraphenylboron (5 microM) was to increase the initial rate of MPP+ uptake by about 20-fold and the steady-state accumulation by about 2-fold. This suggests that there may be a mechanism of efflux of MPP+ from mitochondria which allows MPP+ to cycle across the membrane and thus decrease delta psi. These data indicate that MPP+ interacts with mitochondria independently of its inhibition of NADH-ubiquinone oxidoreductase, and these alternative interactions may be of relevance for its mechanism of neurotoxicity.