- The organs that make up the gastrointestinal tract have high energy demands. Therefore, when these organs vary in mass, they should impact metabolic requirements. Mass of the rumen-reticulum organs, the organs that comprise the largest part of the gastrointestinal tract of ruminants, might vary from bulk or nutrient availability of the diet. We examined differences in mass of the rumen-reticulum organs in white-tailed deer (Odocoileus virginianus (Zimmermann, 1780)) from two sites in Texas, USA, with different diet types. Specifically, at one site deer were fed a pelleted ration and at the other site deer consumed a natural browse diet Accounting for body mass, deer consuming the browse diet had rumen-reticulum organ masses that were about 1.7 times heavier than deer consuming the pelleted diet. Deer consuming the browse diet also had lower diet quality, as indexed by crude protein concentration, than deer consuming the pelleted diet. The digesta loads of deer, however, were similar for the two types of diet. Our study findings are consistent with increased mass of rumen-reticulum organs from greater bulk, not diet quality. Understanding variation in rumen-reticulum organ mass has implications for understanding energy conservation in white-tailed deer.