||This clinical study is composed of two different trials. In the first we investigated and described the perioperative stress response in dogs undergoing elective orchiectomy and ovariohysterectomy. In the second trial we evaluated the efficacy of a commercial dog synthetic appeasing pheromone for the control of the perioperative stress response. The aim of this trial is to describe the behavioural, neuroendocrine, immune and acute phase stress responses in dogs undergoing elective surgery in normal clinical practice conditions. Sixteen dogs were submitted to elective orchiectomy or ovariohysterectomy using a standardized surgical protocol. Each animal was confined to the Intensive Care Unit during pre- and post-surgery, and its perioperative behavioural, neuroendocrine, immune and acute phase responses studied. Behavioural categories, cortisol, prolactin, white blood cells, C-reactive protein and haptoglobin variations were evaluated. Values at different times were compared with basal values shown by the dog in its usual environment. Communicative and explorative behaviours showed high pre-surgery occurrence and were inhibited post-surgery. Decreases in post-surgery activity, interactive behaviours and changes in waking/sleeping patterns were observed. Compared to basal values, cortisol showed a significant increase both during pre- and post-surgery confinement in the ICU cage, being the most sensitive marker of psychological stress. Prolactin values were characterised by a significant decrease at early post-surgery time. The immune response was characterised by long-term neutrophilia and monocytosis, but by short¬term lymphopenia and eosinopenia, limited to early post-operative period. With regard to the acute phase response, both C-reactive protein and haptoglobin showed a post-surgery long-term increase. Changes in behavioural, hematological and biochemical markers showed that perioperative stress represents a major challenge for dog welfare. Surgery, together with its related perioperative procedures, is known to be a major source of stress for dogs. Both psychological and physical stressors activate the behavioural, neuroendocrine, immune and acute phase responses in dogs undergoing elective surgery. A synthetic dog¬appeasing pheromone (DAP) has been marketed to control dog stress response. Its efficacy has been demonstrated recently in many different contexts. We therefore hypothesized that it could also control the perioperative stress response in dogs undergoing elective surgery. For this purpose we studied the behavioural, neuroendocrine, immune and acute phase responses in dogs undergoing elective orchiectomy or ovariohysterectomy. We analyzed the pre- and postoperative variations in behavioural categories, cortisol, glucose, prolactin, white blood cells, haptoglobin and C-reactive protein. A standardised surgery setting was used. Of the results obtained, the behavioural and the prolactin responses showed to be influenced by the DAP treatment. The dogs treated with the synthetic pheromone were more likely to be alert and visually exploring after surgery, and their prolactin response to perioperative stress was significantly lower (P ≤ 0. 05), when compared with the animals receiving the placebo treatment. The HPA axis, immune and acute phase responses were unaffected by the treatment. These results suggest that the dog-appeasing pheromone modifies the behavioural and the neuroendocrine lactotropic perioperative stress responses in dogs undergoing elective surgery. Thus, the use of this product in a clinical setting could improve their recovery and welfare.