Two genes encoding ATP-binding cassette (ABC) transporters were isolated from the crop plant Zea mays (maize). The clones, designated ZmMRP1 and ZmMRP2, were highly homologous to members of the multidrug resistance associated protein (MRP) subfamily. Genomic Southern analysis and characterisation of bacterial artificial chromosome (BAC) clones demonstrated that both genes are present in two copies in maize, which are located in proximity to each other, suggesting the occurrence of duplication events. The full-length genomic and cDNA sequences of ZmMRP1 and 2 were obtained, permitting analysis of the intron/exon structures and protein domains. Intron positions and phasing were conserved between ZmMRP1 and 2 and their closest Arabidopsis homologues. Both clones contained two copies each of the membrane spanning domains and nucleotide-binding folds diagnostic of the ABC superfamily, and ZmMRP1 contained an additional N-terminal membrane-spanning domain (MSD0) that is typical of MRP transporters but which is lacking in the most closely related Arabidopsis and rice MRPs. In contrast, ZmMRP2 and its closest rice but not Arabidopsis homologues lacked MSD0, suggesting the repeated loss of this domain in MRP family evolution. ZmMRP1 and 2 were expressed in all tissues examined but displayed distinct expression profiles in response to herbicide safeners and pro-oxidants. ZmMRP1 was induced by aminotriazole and to a lesser extent by menadione, whereas ZmMRP2 was expressed at a lower constitutive level and did not exhibit strong induction by any of the compounds tested. The characterisation of these clones represents an important step in the experimental analysis of the MRP subfamily in a monocotyledonous crop plant.