Ghrelin is an orexigenic peptide involved in the regulation of energy homeostasis. To investigate the role of ghrelin in the hyperphagia associated with uncontrolled streptozotocin-induced diabetes, food intake was followed in diabetic ghrelin knockout (ghrelin^−/−) and control wild-type (ghrelin^+/+) mice and diabetic Naval Medical Research Institute noninbred Swiss mice treated with either saline or the ghrelin receptor antagonist, d-Lys³-GH-releasing peptide-6 (d-Lys³-GHRP-6) for 5 d. In diabetic ghrelin^−/− mice, hyperphagia was attenuated, and the maximal increase in food intake was 50% lower in mutant than in wild-type mice. The increased food intake observed during the light period (1000–1200 h) in ghrelin^+/+ mice was abolished in mutant mice. Diabetic ghrelin^−/− mice lost 12.4% more body weight than ghrelin^+/+ mice. In diabetic ghrelin^+/+ mice, but not in ghrelin^−/− mice, the number of neuropeptide Y (NPY)-immunoreactive neurons was significantly increased. Diabetic Naval Medical Research Institute noninbred Swiss mice were hyperphagic and had increased plasma ghrelin levels. Treatment with d-Lys³-GHRP-6 reduced daily food intake by 23% and reversed the increased food intake observed during the light period. The change in the number of NPY- (2.4-fold increase) and α-MSH (1.7-fold decrease)-immunoreactive hypothalamic neurons induced by diabetes was normalized by d-Lys³-GHRP-6 treatment. Our results suggest that enhanced NPY and reduced α-MSH expression are secondary to the release of ghrelin, which should be considered the underlying trigger of hyperphagia associated with uncontrolled diabetes.