In vitro activity: Pramipexole, a chemically novel dopamine agonist used for the treatment of Parkinson's disease symptoms, possesses antioxidant activity and is neuroprotective toward substantia nigral dopamine neurons in hypoxic-ischemic and methamphetamine models. Pramipexole reduces the levels of oxygen radicals produced by methylpyridinium ion (MPP+) both when incubated with SH-SY5Y cells and when perfused into rat striatum. Pramipexole also exhibits a concentration-dependent inhibition of opening of the mitochondrial transition pore induced by calcium and phosphate or MPP+. Pramipexole decreases the levels of dopamine metabolites dose dependently, whereas striatal dopamine levels remains unchanged. Pramipexole acts in both of these models to reduce the elevated dopamine turnover and the associated elevation in hydroxyl radical production secondary to increased MAO activity that could be responsible for oxidative damage to the nigrostriatal neurons. Pramipexole (4-100 mM) significantly attenuates DA- or L-DOPA-induced cytotoxicity and apoptosis, an action which is not blocked by D3 antagonist U-99194 A or D2 antagonist raclopride. Pramipexole also protects MES 23.5 cells from hydrogen peroxide-induced cytotoxicity in a dose-dependent manner. Pramipexole can effectively inhibit the formation of melanin, an end product resulting from DA or L-DOPA oxidation in cell-free system.