In primary cultures of embryonic mesencephalic neurons, nitric oxide (NO) facilitates dopamine release in a dose-dependent manner by a Ca+- independent mechanism. In the same neuronal cultures, addition of NO to the incubation medium revesibly inhibited dopamine uptake. Since NO can react with biomolecules characterized by one-electron reduction potentials, such as ascorbate, L-glutathione, O2-, or dopamine, the chemical interaction of NO with dopamine was studied in absence or presence of antioxidants and O2. NO facilitated the formation of dopamine o-semiquinone under aerobic but not anaerobic conditions and was abrogated by the presence of ascorbate or L-glutathione. The inhibition of 3H-dopamine uptake was not abated by the presence of ascorbate or L-glutathione suggesting that NO may interact directly with a cellular target that regulates vesicular storage of dopamine.