A double-barrelled cannula and microdialysis system were used to inject 100 pmoles of neuropeptide Y (NPY) and to collect from the local microenvironment samples of norepinephrine, 3-methoxy-4-hydroxyphenylglycol (MHPG), serotonin (5-HT), 5-hydroxyindole acetic acid (5-HIAA) and dihydroxyphenylacetic acid (DOPAC) a metabolite of dopamine. Following the administration of 100 pmoles of NPY into the ventromedial hypothalamus of conscious rats, there was a significant reduction in the local concentration of norepinephrine and an even greater reduction in the concentration of its metabolite, MHPG, as compared to saline-injected controls. The concentration of serotonin and its metabolite 5-HIAA as well as DOPAC were also reduced in the ventromedial hypothalamus following local injection of NPY. When NPY was injected into the lateral hypothalamus and monoamines measured from areas adjacent to the injection of NPY there was a significant increase in norepinephrine release and a significant increase in the concentration of DOPAC. Serotonin was significantly decreased and there was an increase in the ratio of 5-HIAA to serotonin. NPY is known to stimulate food intake when injected into either area. The present data are consistent with the concept that a reduction in concentration of serotonin is involved in the regulation of food intake following injection of NPY.