The human N-acetyltransferase (Nat2) genetic polymorphisms have been modeled in mouse strains. We determined the phenotype and genotype of the N-acetyltransferase 2 (Nat2*) gene among outbred CD-1 mice and found a mixed population of heterozygous and rapid and slow homozygous genotypes. Phenotypes determined with p-aminobenzoic acid demonstrated complete concordance of slow and rapid genotype and phenotype. The kidney p-aminobenzoic acid/Nat2-acetylating activity of CD-1 female mice showed a 2.5-fold increase at 80 days of age compared with day 1, whereas males showed a 4.3-fold increase at 25 days and a 5.8-fold increase at 80 days. Immunoblot analysis revealed a 2-fold increase in male kidney Nat immunoreactive protein at 80 days of age, whereas no significant differences were detected in female mice. Likewise, the Nat2 mRNA levels determined by ribonuclease protection assay showed an increase in transcript levels in kidney of male mice during postnatal development, whereas they remained unchanged in females. Gender-associated differences of Nat2 activity, protein, and transcript levels were absent in liver. These observations suggest that the increase in Nat2 enzymatic activity in kidney is accomplished by an increase in transcript. We propose that the observed increase in Nat2 transcript expression in male mice may be a result of androgen regulation during development.