Obesity is characterized by increased adipose tissue mass and is often accompanied by a number of other disorders, such as diabetes, hypertension, and hyperlipidemia. To investigate the interrelationship between excessive adipose tissue mass and these associated disorders, we have attempted to reduce adiposity via targeted expression of an attenuated diphtheria toxin A chain to adipose tissue, using the 5'regulatory region of the adipocyte P2 (aP2) gene. Transgenic mice with high levels of toxin expression developed chylous ascites and died shortly after birth. Transgenic mice expressing lower levels of the transgene had normal adiposity and survived to adulthood; however, they showed a complete resistance to chemically induced obesity. Nevertheless, these animals developed hyperlipidemia equal to or greater than their nontransgenic obese littermates. Moreover, MSG-treated transgenic females were fertile, unlike their obese nontransgenic littermates. These data demonstrate the feasibility of gentle manipulation of adiposity and allow a functional dissection of obesity and its metabolic sequelae.