Three steps in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) neurotoxicity were compared with the neurodegenerative effects of the toxin in mice and rats. Firstly, we compared the neurotoxicity of MPTP, mediated by monoamine oxidase (MAO)-B, to that of 1-methyl-4-(2'-methylphenyl)-1,2,3,6-tetrahydropyridine (2'-CH3-MPTP), an analogue oxidized by MAO-A and MAO-B. Both toxins caused degeneration of dopamine terminals in mice but not in rats. In NMRI mice noradrenaline terminals were also affected by both toxins. Pretreatment with deprenyl to prevent MAO-B-mediated oxidation in the capillary endothelium enhanced dopamine toxicity to 2'-CH3-MPTP in nucleus accumbens but no potentiation was seen in striatum and the olfactory tubercle. Secondly, synaptosomal uptake of the 1-methyl-4-phenylpyridinium ion (MPP+) was studied. Uptake in rats was not significantly different from that in the two mice strains. Thirdly, no significant differences were found in MPP(+)-induced lactate production in striatal slices or synaptosomes. We conclude that the lack of effect of MPTP in rats is not due to mechanisms specific for MPTP but probably to the ability of rat catecholamine neurons to cope with, and survive, impaired energy metabolism.