Although the effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in mice have been reported to increase with age, they have not been characterized in the full spectrum of ages. Thus, in spite of a considerable body of scientific literature on the subject, previous reports leave unanswered the question of whether or not the increased susceptibility of fully mature mice is part of the aging process or simply a consequence of maturation. In the present study, the age-related effects of MPTP on striatal dopamine were studied in groups of C57BL/6 mice from young maturity to old age. The major increase in the effects of MPTP occurred between 2 and 10 months of age (equivalent to adolescence and young adulthood in humans). A slight additional increase was observed between 10 and 16 months (young adulthood and middle age) and the dopamine-depleting effects of MPTP significantly declined in truly aged animals (24 months). Of note also is the fact that normal concentrations of striatal dopamine did not decline in the later ages. Additional studies indicated that while neuronal sensitivity to the effects of 1-methyl-4-phenylpyridinium (MPP+; the putative toxic metabolite of MPTP) appears to remain constant, age-related changes in the activity of striatal monoamine oxidase type B (MAO B) paralleled the dopamine-depleting effects of MPTP in the 4 age groups. Indeed, MAO B activity increased between 2 and 16 months and declined slightly, but significantly, between 16 and 24 months. This pattern of age-related changes in MAO B, striatal dopamine and the sensitivity of the nigrostriatal system to toxic insult may provide insights into factors which have been implicated in age-related neurodegeneration and idiopathic Parkinson's disease.