Organic anion transporters (Oats) mediate the initial step of active renal excretion, specifically substrate uptake into proximal tubule cells. Despite extensive characterization of rat Oats, mouse Oat expression patterns are virtually unknown. This study was designed to identify basal expression patterns of mouse Oat1 (Slc22a6), Oat2 (Slc22a7), and Oat3 (Slc22a8) mRNA, compare these patterns with those in rat, and characterize postnatal development of mouse Oat mRNA. Tissues were collected from adult male and female 129J and C57BL/6 mice, and male and female C57BL/6 mice 0 to 40 days of age. Oat mRNA levels were determined by branched DNA signal amplification. Mouse Oat1 mRNA was primarily expressed in kidney of both strains, with male predominance. Mouse Oat2 mRNA levels were highest in kidney of both strains without gender predominance. In both strains, Oat3 mRNA was highest in kidney, and liver expression was male-predominant. However, only 129J mice had higher Oat3 mRNA levels in female kidney than in male kidney. During postnatal development, both Oat1 and Oat2 mRNA levels began to rise after 25 days of age. Oat3 mRNA levels rose gradually from birth through 40 days of age. Oat2 mRNA increased 30-fold during the first 40 days, whereas Oat1 and Oat3 increased about 2-fold. The most notable species differences in Oat mRNA expression were a lack of Oat2 female predominance in mouse kidney and a less dramatic Oat3 male predominance in mouse liver. With the exception of a significant species difference in Oat2 expression, many similarities were found between rat and mouse Oat mRNA levels.