The toxicology and toxicokinetics of a selegiline transdermal system (STS) were evaluated in a 3 month dog study of daily 24 h applications of placebo 4, 8, or 12 STSs in 32 male and 32 female beagle dogs. Each STS delivered approximately 5 mg selegiline over 24 h. No drug-related signs of toxicity were noted in any group with respect to clinical observations, dermal eects, body weight, food consumption, hematology, urinalysis data, or ophthalmoscopic or electrocardiographic examinations. Clinical chemistry data revealed no consistent adverse eects except for an increase in alanine aminotransferase in dogs receiving 8 and 12 STSs. Histological evaluation of tissues revealed the presence of pigment in the Kuper cells of dogs treated with 8 and 12 STSs. There were no pathology ®ndings suggestive of hemolysis or cholestasis. The no-eect level (NOEL) was 4 STSs (2´9 mg kg 71 d 71 ). There were no degenerative or lifethreatening toxic eects up to 12 STSs (8´5 mg kg 71 d 71 ). Gender-related dierences in steady-state plasma levels were not observed. Steady-state plasma concentrations were similar to maximum plasma concentrations obtained in single-dose studies, suggesting that drug accumulation was not evident. Simulation of systemic exposure following oral administration of 16´8 mg kg 71 d 71 from previous toxicology studies indicated that selegiline exposure following 12 STSs is sixfold greater while N-desmethylselegiline, Lamphetamine, and L-methamphetamine exposure is 0´5, 0´15, and 0´14 times the exposure in the oral study. The threefold dierence in NOEL between oral and transdermal studies in the dog (0´8 versus 2´9 mg kg 71 d 71 ) is probably related to greater L-amphetamine and L-methamphetamine exposure following oral administration. The reduction in metabolite formation, relative exposure of selegiline in the dog at the NOEL compared to oral toxicology studies, and margin of safety provided, given that the expected clinical dose is less than the dosage of oral Eldepryl 1 (0´15 mg kg 71 d 71 ), documents the safety of the selegiline drug substance and indicates that additional toxicologic ®ndings with the STS may not be expected. &1997 by John Wiley & Sons, Ltd.