In a combined biochemical and histochemical investigation the following quantitative data on a unilateral nigro-neostriatal dopamine neuron system in the rat have been obtained or calculated. — The number of neurons is 3,500 on the average. — There are about 120 pg dopamine per neuron (1 pg = 10-12 g). — The terminals contain 50 (or 150) times more dopamine than the cell bodies. —The terminal system of one neuron varies mainly between 55 and 77 cm in length and contains about 500,000 varicosities. — The neostriatum, with a volume of 33 mm3, contains 109 dopamine varicosities occupying 0.3 % of the volume, and 4,000,000 nerve cell bodies (not containing dopamine). — One average dopamine varicosity, with a diameter of 0.4 μ, contains 2.5 × 10-4 pg dopamine, and has a dopamine concentration around 8,000 μg/g. — One dopamine cell body of the substantia nigra, with a mean diameter of 30 μ, contains 2.5 (or 0.8) pg dopamine, and has a dopamine concentration around 200 (or 60) μg/g.