OBJECTIVE To evaluate the effect of brachycephaly and body condition score on respiratory thermoregulation of healthy dogs. DESIGN Prospective study. ANIMALS 52 brachycephalic and 53 nonbrachycephalic dogs. PROCEDURES All dogs were exposed to a cool treatment (temperature, 21.8 ± 1.7°C [71.2 ± 3.1°F]; relative humidity, 62.2 ± 9.7%; and ambient enthalpy, 47.7 ± 6.6 kcal/kg) and then a hot treatment (temperature, 32.9 ± 1.7°C [91.2 ± 3.1°F]; relative humidity, 51.9 ± 9.8%; and ambient enthalpy, 74.8 ± 8.7 kcal/kg; heat stress) at least 1 hour later. For each treatment, dogs were allowed to acclimatize to the environment for 15 minutes and then were placed in a sealed whole-body plethysmograph for continuous measurement of the respiratory pattern for 10 minutes. Treatment was discontinued if a dog developed signs of respiratory distress. Respiratory variables and body temperature were compared between the 2 breed types (brachycephalic and nonbrachycephalic) and between treatments. RESULTS Body condition score was positively associated with body temperature independent of environmental conditions or breed type and negatively associated with tidal volume. Brachycephalic dogs had a greater increase in respiratory rate in response to heat stress than did nonbrachycephalic dogs. CONCLUSIONS AND CLINICAL RELEVANCE Results indicated that brachycephalic dogs had decreased capacity for thermoregulation, compared with nonbrachycephalic dogs, but body condition score was a greater determinant of body temperature than breed type. Nevertheless, both upper airway conformation and body condition score should be considered when evaluating whether an individual dog is capable of tolerating heat stress.