The distribution of mRNAs encoding the isoenzymes monoamine oxidase A and B (MAO-A and MAO-B) in monkey locus coeruleus and dorsal raphe nucleus was studied by in situ hybridization histochemistry using 35S-labelled oligodeoxynucleotide probes complementary to cloned human sequences. MAO-A mRNA was highly expressed in noradrenergic neurons of the locus coeruleus while MAO-B mRNA was abundantly and exclusively localized in serotoninergic neurons of the raphe. However, upon emulsion radioautography raphe neurons showed a level of MAO-A mRNA signal noticeably above the background. Our results indicate the utility of human MAO oligodeoxynucleotide probes to identify homologous species of transcripts in the brain of a non-human primate. They also suggest the coexistence of the isoenzymes in raphe neurons as well as the potential role of MAO-A in metabolizing serotonin in vivo.