Conclusions: Selegiline was confirmed as a safe drug with no important side effects in dogs but was not effective for treating canine PDH at the dose used in this study. Due to the availability of other more reliable treatments, it cannot be recommended for treating this disease in dogs. In some dogs, selegiline may have an effect in increasing the activity levels and general alertness, possibly reflecting dopaminergic effects on the brain unrelated to treatment of PDH. It is conceivable that dogs with known pars intermedia tumours could respond better to selegiline, but premortem identification of tumour location in animals is not possible at present with imaging modalities available.