Feeding behavior in response to a carbohydrate preload was determined in lean and obese Zucker rats. Rats were adapted to a schedule of feeding during a 12-h daily dark period. Carbohydrate preloads (cornstarch in water) were given by intragastric intubation 30 min before rats had access to a choice of two diets that differed in their protein and carbohydrate contents. Behavioral responses of lean rats to carbohydrate preloads were prompt and selective. In the first hour of feeding, reduction in intake, mainly of the high carbohydrate-low protein diet, more than compensated for the energy content of the preload. In contrast, obese rats failed to achieve a similar degree of compensation to the energy surplus from the carbohydrate preload during the first hour of feeding. Also, their decreases in food intake were nonselective. In general, it took obese rats two or more hours to adjust their feeding behavior in response to a carbohydrate preload. Increasing the size of the preload did not improve the overall response of obese rats, but an extra 30-min lapse between preload and food access led to selective suppression in first-hour intake from the high carbohydrate-low protein diet. These findings suggest that altered food intake and selection behavior of the obese Zucker rat may be, in part, due to a delay in response to physiologic and metabolic changes arising from carbohydrate ingestion.