Hypothalamic serotonin inhibits food intake and stimulates energy expenditure. High-fat feeding is obesogenic, but the role of polyunsaturated fats is not well understood. This study examined the influence of different high-PUFA diets on serotonin-induced hypophagia, hypothalamic serotonin turnover, and hypothalamic protein levels of serotonin transporter (ST), and SR-1B and SR-2C receptors. Male Wistar rats received for 9 weeks from weaning a diet high in either soy oil or fish oil or low fat (control diet). Throughout 9 weeks, daily intake of fat diets decreased such that energy intake was similar to that of the control diet. However, the fish group developed heavier retroperitoneal and epididymal fat depots. After 12 h of either 200 or 300 μg intracerebroventricular serotonin, food intake was significantly inhibited in control group (21-25%) and soy group (37-39%) but not in the fish group. Serotonin turnover was significantly lower in the fish group than in both the control group (-13%) and the soy group (-18%). SR-2C levels of fish group were lower than those of control group (50%, P = 0.02) and soy group (37%, P = 0.09). ST levels tended to decrease in the fish group in comparison to the control group (16%, P = 0.339) and the soy group (21%, P = 0.161). Thus, unlike the soy-oil diet, the fish-oil diet decreased hypothalamic serotonin turnover and SR-2C levels and abolished serotonin-induced hypophagia. Fish-diet rats were potentially hypophagic, suggesting that, at least up to this point in its course, the serotonergic impairment was either compensated by other factors or not of a sufficient extent to affect feeding. That fat pad weight increased in the absence of hyperphagia indicates that energy expenditure was affected by the serotonergic hypofunction.