The muscular dystrophy with myositis (mdm) mouse carries a deletion in the muscle protein titin. A previous study found that mdm mice shiver at a lower than expected frequency for their body size, are mostly heterothermic with a narrow thermoneutral zone beginning at 34C, and have reduced active muscle stiffness in vivo compared to their wildtype siblings. Impairment in heat production (shivering thermogenesis) could be due to the N2A deletion in the titin protein leading to a more compliant muscle and a lower shivering frequency. The ability of mdm mice to use nonshivering thermogenesis via Uncoupling Protein 1 in brown adipose tissue to generate heat during cold stress may also be impaired and contributing to their heterothermy. The purpose of this study was to evaluate mechanisms for the inability of mdm mice to maintain homeothermy during cold stress and to further understand their metabolic properties. We hypothesized the inability of mdm mice to maintain homeothermy is due to an impairment in heat production (e.g. shivering thermogenesis and nonshivering thermogenesis) and that metabolic rate will reach a maximum at a higher ambient temperature than in wildtype mice. In a temperature experiment, mice were placed in a mouse cage that used indirect calorimetry to measure metabolic rate at four different temperatures (34C, 29C, 24C, 19C). Nonshivering thermogenesis was stimulated by administering 1.2 mg kg−1 of norepinephrine subcutaneously. In the temperature experiment, there was a significant interaction between genotype and temperature, with significant differences in metabolic rates at the lowest temperatures of 19C and 24C (Two‐Way ANOVA and Tukey HSD, p<0.05. Mdm mice had significantly lower body temperatures than wildtype mice at 29C, 24C, and 19C, and reached their maximum metabolic rate at 24C, suggesting that a temperature between 19–24C is the minimum at which mdm mice are able to sustain thermogenesis. Area under the curve (AUC) using the trapezoidal rule was used to calculate total thermogenic capacity following norepinephrine injection. Mdm mice had a lower overall AUC (Welch's T‐Test, p=0.03) as well as a lower overall time that they increased metabolic rate after norepinephrine injection (Welch's T‐Test, p=0.02). Our results suggest that in addition to a deficiency in shivering frequency, mdm mice also have a reduced capacity for nonshivering thermogenesis. The deficiency in shivering thermogenesis and nonshivering thermogenesis combined likely lead to inability to maintain euthermic body temperatures below 34C. Support or Funding Information NSF IOS‐0742483, NSF IOS‐1025806, and NSF IOS‐1456868, the W. M. Keck Foundation, and the Technology Research Initiative Fund of Northern Arizona University. This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.