<jats:title>Abstract</jats:title> <jats:p>Objective: To investigate by behavioral methods the relationship between emotional stress and pain during experimental tooth movement in rats.</jats:p> <jats:p>Materials and Methods: Sixteen male Sprague-Dawley rats (210 to 250 g) were divided into two groups. The experimental group was treated with an active Ti-Ni appliance, and the control group received a passive appliance. A force of 20 gf was delivered by the active appliance between the maxillary first and second molars for 3 days. During this period the rat's behavior was evaluated eight times by means of open-field test and resistance-to-capture test. The specific parameters of animal activity were facial grooming, rearing, and locomotor activity, movement into the center of the open field, and response to capture.</jats:p> <jats:p>Results: Parameters related to stress and pain were higher in the group carrying active appliance, compared to the group with a passive appliance. Statistically significant differences in stress-related behavior between control and experimental groups were found 8 hours after placing the appliance and were most evident on the second day. Pain-related behavior was significantly greater in the experimental group than in the control group at 24 hours.</jats:p> <jats:p>Conclusions: The increase in emotional stress evoked by orthodontic tooth movement may precede the appearance of periodontal pain.</jats:p>