When adult rats were offered isocaloric diet pairs, which differed in carbohydrate contents and which contained the same or differing protein contents, the animals chose between the two foods nonrandomly. The overall ratio of protein to carbohydrate consumed was maintained between 0.20 and 0.43, while the range of possible protein:carbohydrate ratios that the animals could have eaten was much broader, i.e., 0.07-3.60. If diet choice was restricted to pairs containing very large proportions of carbohydrate (average wt/wt % of pair greater than 57%), the regulation of the dietary carbohydrate intake was disturbed: much more carbohydrate was eaten, and the protein:carbohydrate ratio was decreased. These observations suggest that there may be an intrinsic requirement for dietary carbohydrate, and that this requirement may be related to the amount of protein concurrently being consumed. The protein:carbohydrate ratio that the animals choose, when allowed the choice, is also the proportion previously shown to produce neither a decrease nor an increase in brain serotonin levels. It is thus possible that the mechanism by which the brain controls nutrient choice involves alterations in serotonin synthesis, causing foods to be selected that will maintain the release of this transmitter at "optimal" levels.