Abstract Complete subdiaphragmatic vagotomy totally reversed pre-existing VMH lesion induced hyperphagia and obesity independent of the magnitude of the brain lesion effects, suggesting vagal participation in the VMH syndrome. However, complete vagotomy also reduced food and water intake and body weight in brain unlesioned controls. The specificity to VMH lesion obesity of the vagotomy effect is therefore questioned. Complete subdiaphragmatic vagotomy also reversed the obesity produced by prior hypothalamic knife cuts, and prevented the development of knife cut effects in rats which received knife cuts and vagotomies concurrently, or were permitted to recover from the acute effects of vagotomy before receiving knife cuts. Section of the gastric vagus, sparing the hepatic and coeliac branches, had no effect on knife cut-induced obesity, but did reduce water/food ratios and block increases in fasting gastric acid secretion. Thus, upper gastrointestinal effects cannot alone account for the blockade of obesity seen after complete vagotomy. This implicates the coeliac and/or hepatic vagal branches as important to the expression of hypothalamic obesity.