We recently reported that intraperitoneal administration of leptin-related synthetic peptide 116-130 [LEP-(116-130)] resulted in reduced food intake and significant weight loss in homozygous female C57BL/6J ob/ob mice. In this study, we used two in vitro bioassays to show that the interaction of LEP-(116-130) with the long form of the leptin receptor (OB-Rb), the receptor isoform that is predominantly expressed in the hypothalamus, is not required for the observed in vivo effects of the peptide on energy balance. LEP-(116-130) was unable to compete the binding of alkaline phosphatase-leptin fusion protein to OB-R. Moreover, LEP-(116-130) was unable to activate signal transduction by OB-Rb in vitro. In homozygous female C57BLKS/J-m db/db mice that do not express OB-Rb, intraperitoneal administration of LEP-(116-130) reduced body weight gain and blood glucose levels but not food intake, which further supports a mechanism of action that does not require peptide interaction with OB-Rb.