Acetoacetyl-CoA synthetase (AACS, acetoacetate-CoA ligase, EC 6.2.1.16) is a novel cytosolic ketone body (acetoacetate)-specific ligase, the physiological role of which remains to be elucidated. We examined the expression profiles of AACS mRNA in adult rat tissues, finding that it was particularly abundant in male subcutaneous white adipose tissue after weaning. In white adipose tissue, AACS mRNA was preferentially detected in mature adipocytes but not in preadipocytes. The AACS mRNA expression in primary preadipocytes increased during the adipocyte differentiation. These expression profiles were similar to that of acetyl-CoA carboxylase-1, but not like to that of 3-hydroxy-3-methylglutaryl-CoA reductase. These results suggest that AACS in adipose tissue plays an important role in utilizing ketone body for the fatty acid-synthesis during adipose tissue development.