We have previously generated a conditional floxed mouse line to study androgen action, in which exon 3 of the androgen receptor (AR) gene is flanked by loxP sites, with the neomycin resistance gene present in intron 3. Deletion of exon 3 in global AR knockout mice causes androgen insensitivity syndrome (AIS), characterized by genotypic males lacking normal masculinization. We now report that male mice carrying the floxed allele (AR^lox) have the reverse phenotype, termed hyperandrogenization. AR^lox mice have increased mass of androgen-dependent tissues, including kidney, (p<0.001), seminal vesicle (p<0.001), levator ani muscle (p=0.001), and heart (p<0.05). Serum testosterone is not significantly different. Testis mass is normal, histology shows normal spermatogenesis, and AR^lox males are fertile. AR^lox males also have normal AR mRNA levels in kidney, brain, levator ani, liver and testis. This study reaffirms the need to investigate the potential phenotypic effects of floxed alleles in the absence of cre in tissue-specific knockout studies. In addition, this androgen hypersensitivity model may be useful to further investigate the effects of subtle perturbations of androgen action in a range of androgen-responsive systems in the male.