Explants of embryonic rat mesencephalon were grown in organotypic culture. Addition of 10 microM 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to the culture medium for 4 to 7 days resulted in loss of dopamine cell bodies and fiber outgrowths, as observed by fluorescence histochemistry. At the same time, the cultures showed decreased uptake of tritium-labeled dopamine. However, no signs of generalized toxicity were evident when the explant cultures were viewed by light and phase-contrast microscopy. These results show that MPTP exerts a relatively selective destructive action in dopamine neurons in vitro, similar to the action observed in humans and monkeys in vivo. Pargyline (10 microM), a monoamine oxidase inhibitor, protected the dopamine neurons in the explants. Organotypic cultures provide an experimental model for the study of the properties of MPTP in vitro.