This study examined the metabolic effects of weight cycling, i.e., repeated periods of weight loss followed by regain. There were three groups of adult, male Sprague-Dawley rats: (1) Chow Controls (a normal weight control group fed chow throughout); (2) Obese Controls (animals fed a high-fat diet throughout); and (3) Obese Cycling (obese animals cycled through two bouts of caloric restriction and refeeding). The cycled animals showed significant increases in food efficiency (weight gain/kcal food intake) in the second restriction and refeeding periods compared to the first, i.e., weight loss occurred at half the rate and regain at three times the rate in the second cycle. Several physiological changes were associated with this cycling effect. At the end of the experiment, cycled animals had a four-fold increase in food efficiency compared to obese animals of the same weight who had not cycled. These data suggest that frequent dieting may make subsequent weight loss more difficult. The possible metabolic and health consequences of "yo-yo" dieting are discussed.