Analysis of the mouse genome has revealed eight multidrug resistance-associated (Mrp) transporters, with mouse homologs for all human MRPs except MRP8. Whereas MRP expression in tissues of humans and rats has been examined, no characterization exists for mice. Furthermore, the ontogeny of mouse Mrps is unknown, and such knowledge may be helpful in understanding age-related pharmacokinetics. Therefore, the purpose of this study was to quantitatively determine 1) expression of the Mrp family in 12 different tissues, 2) gender variations in Mrp expression in liver and kidney, and 3) whether Mrp expression is altered during development. Highest expression of the Mrp family members is as follows: Mrp1 in testes, ovary, and placenta; Mrp2 in intestine, followed by liver and kidney; Mrp3 in large intestine; Mrp4 in kidney; Mrp5 in brain, followed by lung and stomach; Mrp6 in liver; Mrp7 in testes, intestine, and kidney; and Mrp9 solely in testes. Gender differences in Mrp expression were observed: Mrp1, 3, and 4 in kidney, as well as Mrp1 and 4 in liver were female-predominant. Ontogeny of the four Mrps expressed in liver was as follows: Mrp2 and Mrp4 were expressed at adult levels at birth; Mrp3 reached adult levels at day 30, and Mrp6 was not expressed until day 10. In kidney, Mrp1 and Mrp5 were expressed at adult levels at birth, whereas Mrp2, 3, 4, and 6 generally increased over time. In conclusion, marked differences in expression of the individual Mrp family members exist in various tissues, with age, and with gender.