Grazing muzzles are highly effective at reducing forage intake in horses and are a popular tool to control horse weight. However, grazing muzzle design may cause horses stress. The objective of these studies was to determine how grazing muzzles impact behavior and physiological stress in grazing horses. Two groups of 6 miniature horses, housed individually or in a herd, wore grazing muzzles for 0, 10, and 24 h/d. Over 9 weeks, body weight, heart rate parameters, salivary cortisol concentrations, and observations of behavior were collected. Results indicate muzzling did not seem to cause physiological stress as measured by cardiac and salivary cortisol parameters but did alter grazing and locomotive patterns. Muzzling for 24 h/d was necessary for weight loss and was associated with lower heart rate and higher heart rate variability. These findings suggest that muzzles do not cause stress in horses, even if left on for 24 h/d.