In this study, the authors sought to evaluate the mechanisms responsible for echocardiographically determined differences in cardiac structure and function between conscious and anesthetized mice to determine whether such differences were more or less evident in diseased states. Cardiac parameters were determined by transthoracic echocardiography. Mice anesthetized with a mixture of ketamine and xylazine showed reductions in heart rate (HR, 252 +/- 16 beats/min versus 734 +/- 9 beats/min) and fractional shortening (FS, 35% +/- 2% versus 59% +/- 2%) compared with conscious mice. Conscious mice responded little to the beta-agonist isoproterenol or atropine, but showed profound reductions in HR and FS in response to the beta(1)-antagonist atenolol. In contrast, both isoproterenol and atropine led to increases in HR and FS in anesthetized mice. The stress in conscious animals was reduced by the sedative midazolam, leading to partial restoration of responses to isoproterenol. Mice with constitutive activation of the beta-adrenergic system, due to cardiac overexpression of beta(2)-adrenergic receptors or with heart disease (myocardial infarct and pressure-overload hypertrophy) showed few differences in functional parameters between conscious and anesthetized states, attributable to pre-existing activation of the sympathetic and beta-adrenergic systems, even during anesthesia. The results indicate that the autonomic nervous system plays a critical role in the observed differences in cardiac structure and function between anesthetized and conscious mice.