The relationship between carbohydrate and protein consumption and brain serotonin levels was investigated using Sprague-Dawley rats trained to press two levers. Each response on one lever was followed by a carbohydrate-rich 45-mg food pellet and each response on another lever was followed by a 45-mg pellet relatively high in protein. Access to the carbohydrate pellets immediately prior to the daily session depressed the number of carbohydrate pellets consumed but had little effect on the number of protein pellets consumed. Prior access to standard rat chow, which is relatively high in protein, had the opposite effect. Prior access to carbohydrates in solution did not affect pellet consumption. Plasma amino acid levels and brain serotonin levels were altered by intraperitoneal injections of insulin (2.4 U/kg) and tryptophan (40 mg/kg), but consumption of carbohydrate pellets relative to protein was not changed by these injections. Saline and glucose injections did not change relative consumption either. The results emphasize the importance of oral-sensory cues in food intake and selection but are not supportive of a major role for serotonin.