now wondered if parkinson's could be, at least in part, a modern disease, triggered by toxins like mptp. >> i think it's a very exciting time for research in this disease. and for patients with parkinson's disease, and their families, that's good news. thank you very much. we had real hope that this would solve the disease. and i thought, three to four years we would have the answer. in fact, in three to four years, we did have an answer, and that was that mptp itself was not the cause of parkinson's. >> narrator: not the cause, but a key clue. indeed, recent studies have built on langston's initial breakthrough, showing that pesticide exposure can increase disease risk by as much as 70%. and many scientists now think genes and the environment might actually work hand in hand to cause most parkinson's, with certain genes triggering the disease only when a toxic exposure takes place. there's an old saying that genetics loads the gun and environment pulls the trigger, and that may be the case in parkinson's. we still don't have a smoking gun, that's for sure, but that's what we're looking

and then both groups arean injected with the toxin mptp, the same toxic substance that caused instant parkinson's in the frozen addicts. >> what we're seeing is the monkeys that have run on the treadmill have very little effect of the mptp. they're still able to use the side in which they received the injection, and they can use it very normally. the monkeys that are running look very different from the sedentary controls. and the sedentary controls aren't really using their arm. they're letting it hang down. it makes us think that exercise somehow changes how the brain is functioning, and protects it against damage. >> everybody ready? >> ready. >> narrator: to see exactly what's going on the animals are then given a brain scan. the results prove to be dramatic. >> it's a pretty massive lesion. >> yeah. >> narrator: the mptp injection has decimated dopamine production in the monkeys who watched but didn't run. >> but in contrast, the running animal... >> the lesion is much less. >> in fact in some of these slices you'd be hard pressed to know that there is a lesion at all. >> right,