An androgen receptor (AR) null mutant mice line as generated by means of a Cre-lox P system. The male (AR(L-Y)) (KO) mice exhibited typical features of testicular feminization mutant (Tfm) disease in external reproductive organs with growth retardation. The growth curve of the male AR KO mice as similar to that of the wild-type female littermates until the 10th week of age, but thereafter a drastic increase in the growth as observed with development of obesity. Clear increase in the wet freights of white adipose tissues. but not of brown adipose tissue, was found in the 30-week-old male AR KO mice. However, no significant alteration in serum lipid parameters and food intake was observed. Thus. the present results suggest that AR may serge as a negative regulator of adipose development in adult males. (C) 2002 Elsevier Science (USA). All rights reserved.