In previous studies and in the accompanying paper, 1-methyl-4-(2'-methylphenyl)-1,2,3,6-tetrahydropyridine (2'CH3-MPTP) was found to be more potent than MPTP in producing dopaminergic neurotoxicity in mice. One purpose of the present study was to determine whether 1-methyl-4-(2'-methylphenyl)pyridinium (2'CH3-MPP+), the primary oxidation product of 2'CH3-MPTP both in vivo and in vitro, inhibits mitochondrial respiration as does 1-methyl-4-phenylpyridinium (MPP+), the primary oxidation product of MPTP. Another aim was to determine whether treatments which modify MPTP- and 2'CH3-MPTP-induced neurotoxicity in vivo also cause parallel changes in the metabolic toxicity of these compounds. It was found that 2'CH3-MPP+, like MPP+, inhibited the oxidation of NAD(H)-linked substrates by isolated brain mitochondria in a concentration- and time-dependent manner, whereas succinate oxidation was not affected. Thus, the effect was on Complex I in the electron transport chain. Furthermore, 2'CH3-MPP+, like MPP+, enhanced lactate formation by neostriatal tissue slices as would be expected if Complex respiration were inhibited. MPP+ was slightly more potent than 2'CH3-MPP+ in both of these studies. However, 2'CH3-MPTP was several-fold more potent than MPTP in increasing lactate accumulation by the neostriatal slices. This difference in potency correlated with the differing capacities of 2'CH3-MPTP and MPTP to be oxidized by monoamine oxidase (MAO).(ABSTRACT TRUNCATED AT 250 WORDS)