Central to several current theories of the etiology of Parkinson's disease is the premise that the nigrostriatal dopaminergic system degenerates with normal aging. Much of the evidence for this assertion has come from postmortem neurochemical studies. We have used L-6-[18F] fluoro-Dopa and positron emission tomography in 26 healthy volunteers (age range, 27-76 years) to examine striatal and frontal cortical tracer uptake. Data have been analyzed by using a graphical approach to calculate an influx constant (Ki) for L-6-[18F]fluoro-Dopa uptake into the caudate, putamen, and medial frontal cortex of each subject. In the population studied, there was no decline in Ki with age for any of these structures. A series of physiological measurements made on the older subjects also showed few significant changes with age. The positron emission tomographic findings demonstrate preservation of nigrostriatal dopaminergic function in normal aging. The pathological process causing Parkinson's disease may operate closer to the time of presentation than has been suggested.