Guinea pigs were injected subcutaneously with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in maximal tolerated doses (8 mg/kg, once daily) for 10 or 15 days. No neurological effects were noted, other than sedation and hypotonia lasting a few hours after each injection, either in animals maintained on normal diet or in animals fed an ascorbate-deficient diet and rendered severely scorbutic. Subsequent chemical analyses of the striatum showed no evidence of lasting damage to nigrostriatal dopaminergic neurons in MPTP treated guinea pigs on normal diet, and minimal evidence of permanent damage to these neurons in scorbutic animals. MPTP was undetectable in the urine of MPTP-treated animals, although a metabolite, presumably 1-methyl-4-phenylpyridinium ion (MPP+) was regularly present in urine. The relative lack of neurotoxicity of MPTP in the guinea pig remains unexplained. This species clearly is not a suitable small animal for MPTP-induced parkinsonism.