Weanling rats, with electrolytic lesions primarily destroying the ventromedial hypothalamic nuclei (V.M.N.) and sparing the median eminence, have in the past been shown to be growth-retarded, to have reduced pituitary and plasma growth hormone levels, to be obese despite normal body weight gains, and to show normophagia, normoglycemia, hyperinsulinemia, hypertriglyceridemia, and hypercholesterolemia. In the present study, the spontaneous running activity of such animals, as measured in rotating drum type activity cages, was normal during the first 24 h after the operation, became suggestively reduced during the 1st week, and was significantly reduced 2 weeks after the hypothalamic operation. Food intake was normal throughout the experiment but the V.M.N. lesioned animals weighed less than the controls despite a higher obesity index. It is concluded that hypoactivity may be a contributing factor in the development of the normophagic–normoglycemic–hyperinsulinemic obesity that characterizes the weanling rat ventromedial syndrome.