Six hundred forty-one dogs with clinical signs consistent with canine cognitive dysfunction syndrome (CDS) were treated orally with selegiline hydrochloride at 0.5 to 1.0 mg/kg once daily for 60 days. Response to selegiline treatment on days 30 and 60 were similar. On day 60, 77.2% of dogs showed an overall improvement; response to treatment by clinical sign ranged from 67.8% (activity or sleep/wake cycle) to 77.8% (disorientation and interaction with family members). All dogs enrolled in the study were monitored for possible adverse events; diarrhea (4.2%), anorexia (3.6%), and vomiting/salivation (3.4%) were noted most frequently. Results of this study indicate the majority of the dogs with CDS responded to treatment with Anipryl by day 30.