Analysis of the nasal region in fossils of three theropod dinosaurs (Nanotyrannus, Ornithomimus, and Dromaeosaurus) and one ornithischian dinosaur (Hypacrosaurus) showed that their metabolic rates were significantly lower than metabolic rates in modern birds and mammals. In extant endotherms and ectotherms, the cross-sectional area of the nasal passage scales approximately with increasing body mass M at M(0.72). However, the cross-sectional area of nasal passages in endotherms is approximately four times that of ectotherms. The dinosaurs studied here have narrow nasal passages that are consistent with low lung ventilation rates and the absence of respiratory turbinates.