Short-term treatment of lean and obese mice with the fatty acid synthase (FAS) inhibitor, C75, alters expression of hypothalamic neuropeptides thereby reducing food intake, body weight, and body fat. Here we report the long-term effects of C75 on obese (Ob/Ob) mice. A low dose of C75 administered every third day for 30 days reduced food intake by 62% and body weight by 43% whereas body weight of ad lib-fed controls increased by 11%. Loss of body weight correlated with decreased adipose and liver tissue mass. Decreased food intake correlated with decreased expression of hypothalamic neuropeptide mRNAs for NPY, AgRP, and MCH and an increased expression of neuropeptide mRNAs for alphaMSH (i.e., POMC) and CART. Consistent with increased energy expenditure, C75 treatment caused greater weight loss than pair-fed controls and increased expression of skeletal muscle UCP-3 mRNA. Lowered blood glucose was due largely to restriction of food intake. C75 blocked the normal fasting-induced rise in blood free fatty acids and ketones due either to decreased adipose tissue lipolysis and hepatic ketogenesis or increased fatty acid and ketone utilization by peripheral tissues, notably skeletal muscle.