d-Fenfluramine (0.63 mg/kg i.p.), a serotonin (5-hydroxytryptamine, 5-HT) releaser and re-uptake inhibitor, reduced the eating caused by neuropeptide Y (235 pmol) injected into the paraventricular nucleus of the hypothalamus. The 5-HT1 and 5-HT2 receptor antagonist metergoline (1.0 and 2.0 mg/kg i.p.) and the 5-HT1A and 5-HT1B receptor antagonist (+/-)-cyanopindolol (3.0 and 8.0 mg/kg s.c.) significantly antagonized the effect of d-fenfluramine. The 5-HT2A and 5-HT2C receptor antagonist mesulergine (0.1 and 0.3 mg/kg s.c.) and the 5-HT2A receptor antagonist ketanserin (2.5 and 5.0 mg/kg i.p.) did not significantly modify the effect, nor did the 5-HT1A and 5-HT1B receptor antagonist (-)-propranolol (20-40 nmol), injected bilaterally into the paraventricular nucleus of the hypothalamus. The results suggest that d-fenfluramine reduces neuropeptide Y's hyperphagia by indirectly stimulating 5-HT1B receptors outside the paraventricular nucleus of the hypothalamus.