Ondansetron, a selective serotonin-type 3 (5-HT₃) receptor antagonist, was used to test the hypothesis that duodenal infusion of isosmotic solutions of Polycose or its hydrolytic product glucose, suppressed intake through 5-HT₃ receptors. Polycose suppressed sucrose intake across both concentrations infused (132mM, 7.6 ± 0.6 ml; 263mM, 2.3 ± 0.5 ml), compared to intake under control conditions (12.6 ± 0.3 ml, P <0.001). Pretreatment with 1.0 mg/kg ondansetron attenuated reduction of sucrose intake induced only by the highest concentration of Polycose (4.6 ± 0.8 ml, P=0.004). Dose response testing revealed that suppression of food intake by 263mM Polycose was attenuated by ondansetron administered at 1.0, 2.0, and 5.0mg/kg equally, but not when given at 0.125, 0.25, and 0.5 mg/kg. Acarbose, an alpha-glucosidase inhibitor, attenuated Polycose-induced suppression of food intake and pretreatment with 1.0 mg/kg ondansetron had no further effect. Suppression of intake following 990mM glucose, but not mannitol infusion, was attenuated by pretreatment with 1.0 mg/kg ondansetron. The competitive SGLT₁ inhibitor, phloridzin had no effect on 60 min 990mM glucose-induced suppression of intake or the ability of ondansetron to attenuate this suppression of intake. Conversely, glucose-induced suppression of intake was attenuated by phloridzin at earlier time points, and further attenuated when rats were pretreated with 1.0 mg/kg ondansetron. Ondansetron administration alone had no effect on intake at any dose tested. We conclude that 5-HT₃ receptors participate in the inhibition of food intake by intraduodenal infusion of carbohydrate solutions through a post-hydrolytic, preabsorptive mechanism.