8-Hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), a 5-hydroxytryptamine (5-HT)-1A selective agonist was used to investigate a possible role of somatodendritic serotonin-1A receptors in the precipitation of hyperphagia and decreases of 5-HT metabolism associated with long-term consumption of sugar rich-diet. In the first part of study, dose-related hyperphagic effects of 8-OH-DPAT were monitored in freely feeding rats. In the second part of study, rats were fed freely on a sugar-rich diet (prepared by mixing standard rodent diet with table sugar in the ratio of 3:1 [w/w]) for 5 weeks. Hyperphagic effects of 8-OH-DPAT were monitored in sugar-rich diet and normal diet treated rats by injecting the drug at a dose of 0.25 mg/kg body weight, a dose that produced significant hyperphagia. Effects of 8-OH-DPAT on decreasing 5-HT metabolism in the hypothalamus were also investigated in the two groups. Results showed that administration of 8-OH-DPAT at a dose of 0.25 mg/kg body weight elicited hyperphagia and decreased 5-HT metabolism in normal diet treated animals but the effects in sugar-rich diet treated animals were smaller and not significant suggesting a decrease in the effectiveness of somatodendritic 5-HT-1A receptors, which provide a feedback control over the synthesis and release of 5-HT in terminal region. A possible mechanism involved in sugar-diet induced decreases of 5-HT metabolism is discussed.