From studies on sympathetically innervated peripheral tissues it is well known that both neuronal and non-neuronal transport systems contribute to the inactivation of released monoamine transmitters. The close proximity between synapses and glia cell processes in the CNS leads to the so far unresolved question whether non-neuronal transporters are involved in the inactivation of centrally released monamine transmitters such as noradrenaline, dopamine and 5-hydroxytryptamine. 1-Methyl-4-phenylpyridinium (MPP+) is a prototypical substrate of the extraneuronal monoamine transporter (uptake2). [3H]MPP+ was found to accumulate in various human glioma cell lines. [3H]MPP+ transport was characterized in more detail in HTZ146 human glioma cells. The Ki values of various compounds for the inhibition of initial rates of [3H]MPP+ transport into HTZ146 cells were closely correlated with known Ki values for the inhibition of the extraneuronal monoamine transporter (P < 0.01, r = 0.991, n = 7). The rank order of inhibitory potencies was decynium 22 > corticosterone > cyanine 863 > O-methylisoprenaline > quinine > clonidine > quinidine. [3H]MPP+ accumulation was investigated not only in various CNS tumour cell lines but also in primary cultures of human astrocytes and rat cerebral cortex slices. In all tested experimental systems, accumulation was sensitive to cyanine-related inhibitors of the extraneuronal monamine transporter. These findings suggest that the extraneuronal monamine transporter exists in glia cells. Furthermore, it was shown that MPP+ is able to make use of the extraneuronal monoamine transporter not only to enter but also to leave glia cells. This finding suggests that the extraneuronal monoamine transporter may play a key role in the mechanism of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) neurotoxicity.