The efficacy of amitriptyline as a pharmacological adjunct to behavioral modification in the clinical management of aggressive behaviors in dogs was evaluated in two phases. Twelve dogs presenting for aggressive behaviors were treated sequentially with amitriptyline (2 mg/kg body weight, per os [PO] bid) and a placebo for 4 weeks in a prospective, randomized, double-blind, placebo-controlled trial. Standardized protocols for behavior modification were implemented throughout the trial. Owners maintained behavioral records and reported on the number of aggressive incidents as well as the dog's overall improvement at the end of each 4-week period. In the second phase, 27 cases of dogs presenting for aggressive behaviors and treated with amitriptyline were reviewed, and clients were contacted to record each dog's response to treatment. Reports were compared to those for dogs receiving behavior modification alone (i.e., placebo phase of prospective study). No significant difference was observed in the patients' responses to adjunctive amitriptyline versus behavior modification alone.