The normal maturation of biliary organic anion excretion in newborn rats can be enhanced by microsomal enzyme-inducing chemical treatment, yet the mechanism for this phenomenon is not known. Multidrug Resistance Protein 2 (Mrp2) is a biliary efflux transporter that is inducible by select microsomal enzyme-inducing chemicals. Thus, the aims of this study were to compare the normal and pregnenolone-16alpha-carbonitrile (PCN)-induced postnatal ontogeny of Mrp2 in male and female rats. Mrp2 protein increased in an age-dependent manner in both sexes between 0 and 90 days of age. At birth, Mrp2 protein in both male and female rats was the same, approximately 70% of adult levels. Mrp2 protein in both sexes reached maximal expression levels that were higher than adult levels (male: days 25-40; female: day 45), then decreased to adult levels, at which age Mrp2 protein expression in male and female rats was the same. Second, male and female rats of various ages were treated with PCN (75 mg/kg, ip) or corn oil for 4 days, after which livers were removed and analyzed for Mrp2 protein and mRNA expression. PCN accelerated the expression of Mrp2 protein in male and female rats as early as 10 days of age, whereas, PCN did not affect male and female Mrp2 mRNA ontogeny. These data suggest that PCN increased Mrp2 protein by a sex-independent posttranscriptional mechanism.