Studies were undertaken to analyze the role of moisture content of foods in producing dietary obesity. Female CD rats consumed more energy when offered a sucrose solution and plain water to drink than when they were only given plain water, regardless of the sugar content of their diet (0-65%). This suggested that the overeating that commonly occurs when sucrose solutions are offered may not be due to sucrose per se. In subsequent experiments, rats were fed modified AIN-76 diets high in sucrose, starch or fat for 28-42 d. For some rats, the diet was liquefied by adding water to make a 32% suspension. Plain drinking water was always available. Rats fed high carbohydrate liquid diets, with or without solid diet, consumed 8-15% more energy than rats fed solid diet only. Rats fed liquid diets also gained 43-206% more weight than did rats fed solid diets. Analysis of carcass composition revealed that the liquid diets increased body fat. For high fat diets, the results were more complicated. Addition of water to a low cellulose, high fat diet did not increase adiposity, whereas addition of water to a high cellulose, high fat diet did increase adiposity. These results suggest that the obesity-inducing effects of feeding sugar solutions or cafeteria diets may be due, in part, to the high water content of these foods.