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In rodents, weight reduction after peptide YY[3-36] (PYY[3-36]) administration may be due largely to decreased food consumption. Effects on other processes affecting energy balance (energy expenditure, fuel partitioning, gut nutrient uptake) remain poorly understood. We examined whether s.c. infusion of 1 mg/(kg x d) PYY[3-36] (for up to 7 d) increased metabolic rate, fat combustion, and/or fecal energy loss in obese mice fed a high-fat diet. PYY[3-36] transiently reduced food intake (e.g., 25-43% lower at d 2 relative to pretreatment baseline) and decreased body weight (e.g., 9-10% reduction at d 2 vs. baseline) in 3 separate studies. Mass-specific metabolic rate in kJ/(kg x h) in PYY[3-36]--treated mice did not differ from controls. The dark cycle respiratory quotient (RQ) was transiently decreased. On d 2, it was 0.747 [+ or -] 0.008 compared with 0.786 [+ or -] 0.004 for controls (P < 0.001); light cycle RQ was reduced throughout the study in PYY[3-36]--treated mice (0.730 [+ or -] 0.006) compared with controls (0.750 [+ or -] 0.009; P < 0.001). Epididymal fat pad weight in PYY[3-36]--treated mice was ~50% lower than in controls (P < 0.01). Fat pad lipolysis ex vivo was not stimulated by PYY[3-36]. PYY[3-36] decreased basal gallbladder emptying in nonobese mice. Fecal energy loss was negligible (~2% of ingested energy) and did not differ between PYY[3-36]--treated mice and controls. Thus, negative energy balance after PYY[3-36] administration in diet-induced obese mice results from reduced food intake with a relative maintenance of mass-specific energy expenditure. Fat loss and reduced RQ highlight the potential for PYY[3-36] to drive increased mobilization of fat stores to help meet energy requirements in this model. KEY WORDS: * obesity * metabolic syndrome * gut peptide * lipid * [beta]-oxidation