The possibility of imaging monoamine oxidase (MAO) containing neurons through the MAO-mediated conversion of the nonfluorescent tetrahydropyridine compoundtrans-1-methyl-4-[4-dimethylaminophenylethenyl]-1,2,3,6-tetrahydropyridine (t-THP) to the corresponding fluorescenttrans-1-methyl-4-[4-dimethylaminophenylethenyl]pyridinium species (t-P+) was examined with the aid of human neuroblastoma cells (SH-SY5Y). Fluorescence microscopy and fluorescence measurements established the intracellular formation of a fluorescent species with maximal excitation/emission wavelengths of 485/620 and 530/620 nm corresponding to the fluorescence characteristics of synthetict-P+. An independent assay confirmed the presence of both MAO-A and MAO-B in these cells. As expected, the development of the fluorescence was inhibited by both clorgyline (an MAO-A inhibitor) and deprenyl (an MAO-B inhibitor). Cytotoxic effects, as determined by trypan blue dye exclusion for viability and by the MTT [3-(4,5-dimethylthiazole-2-yl)-2,5-diphenyltetrazolium bromide] assay for mitochondrial integrity, were not observed in cells incubated with concentrations oft-THP as high as 10−3Mfor 4 hr. The results from these studies with a neuronal cell line of human origin suggest: (1) that SH-SY5Y cells metabolize and, therefore, can be used for study of tetrahydropyridine compoundsin vitro,and (2) thatt-THP may be a useful agent to monitor neurodegenerative processes in MAO-rich neurons, including the dopaminergic nigrostriatal neurons that are damaged by the parkinsonian-inducing tetrahydropyrridine MPTP. The potential advantage of usingt-THP over related imaging techniques is the possibility of assessing neuronal function by anin vivoprocessing of the reporter molecule rather than by postmortem immunofluorescent or formaldehyde-based procedures.