The neurotransmitter dopamine has biological attributes that make it amenable to study by positron emission tomography, unlike many of the 40 or so neurotransmitters that have been identified in the brain. Dopamine deficiency in the nigrostriatal system is a characteristic of Parkinson's disease, and a disturbance of dopamine metabolism is still widely held to be responsible for the syndrome of schizophrenia. Despite its importance in the regulation of locomotion and mood, it has been impossible to visualize the intracerebral distribution of dopamine and measure its regional metabolism in man. In the first demonstration of the regional distribution of a neurotransmitter in the brain of conscious normal man, we show here that L-3,4-dihydroxyphenylalanine (L-dopa) labelled in the 6-position with the positron-emitting radionuclide fluorine-18, localizes specifically in the dopaminergic pathways of the human brain where its turnover could be measured atraumatically by positron emission tomography.