To clarify neuronal disturbance in the hypothalamus reflecting the development of obesity in ventromedial hypothalamic nucleus (VMH)-lesioned rats, we investigated the contents of neurotransmitters in the hypothalamus after pretreatment by microwave irradiation, contents of neurotransmitter metabolites in third ventricle fluid and catecholamine contents in the adrenal gland. The hypothalamic contents of norepinephrine (NE) and dopamine (DA) were selectively decreased, but acetylcholine (ACh) and serotonin (5-HT) levels were not changed from those in controls. In the lateral part of the hypothalamus, also, a significant decrease of NE content was detected. On the other hand, NE and DA metabolites, MHPG, DOPAC and HVA were increased in the third ventricle fluid in VMH lesion-induced obese rats. Wet weight and content of epinephrine in the adrenal gland were decreased, though the content of NE was preserved. These results indicate that a disturbance of NE and DA neurons in the hypothalamus is involved in the development of VMH lesion-induced obesity. In addition, an increment of the activities of NE and DA systems in the central nervous system as a whole and some irregularity in the sympatho-adrenal system might contribute to VMH obesity.