Cheek pouch volumes (V in cm3) were positively and significantly related to body mass (M in g) in 12 species and 14 populations of heteromyid rodents by the relationship V=0.065 M 0.887. When genera were considered separately, Microdipodops, Perognathus, and Thomomys conformed closely to the relationship, but Dipodomys did not. All species could obtain sufficient energy to meet their daily requirements from one maximum cheek pouch load, but the larger Dipodomys and Thomomys can carry a greater amount of energy relative to their needs. It is postulated that Thomomys and herbivorous Dipodomys conform to the relationship because they must transport food of low density and nutritional value; other Dipodomys, which feed on seeds of high density and greater nutritional worth, appear to have passed a threshold in size beyond which conformance to an allometric relationship is unnecessary. Thus, the two most important factors governing cheek pouch capacity are body mass and the density of the preferred food.