This study was designed to investigate the toxicity of both MPTP and MPP+ using some simple cell systems, such as PC12 and C6 cultures, as models. Exposure of PC12 cells to 0.5 mM MPTP for 72 h resulted in a 50% cell loss with respect to the control cells, and clorgyline, a MAO-A inhibitor, antagonized this toxic effect. Higher concentrations of MPTP demonstrated only a weak cytostatic effect on C6 cells. Moreover, MPP+ showed a toxic effect which was 100 times more evident than MPTP toxicity in the PC12. We found a single, saturable class of [3H]MPP+ binding sites with a relatively high affinity both in PC12 and C6 cell lines. Moreover, the most susceptible cell line towards the toxic effects of both MPTP and MPP+, i.e. PC12, has the higher number of MPP+ binding sites. Our results suggest that MPTP can be toxic not only via MAO-B, but also via MAO-A activity and we propose PC12 as a model to study the intracellular mechanisms of MPTP and MPP+ toxicity.