The pharmacokinetics characteristics of propranolol (PPL) in horses was studied by administering the drug intravenously or orally to the animals. The predominant primary pathway was ring oxidation, and 4-hydroxypropranolol glucuronide (4-OHPG) was the major metabolite in both plasma and urine. Side-chain glucuronidation and oxidation were not significant. A two-compartment model was employed for PPL followed by a one-compartment model for 4-OHPG. After oral administration, one-step absorption and two-step first pass metabolism were employed. The fraction absorbed of PPL was approximately 70% after oral administration, and the bioavailability varied among individual horses from 1 to 79% depending on the first pass metabolism. The biologic half-life (T_1/2) of PPL obeys the allometric equation in some animal species including rats and horses, except for human. T_1/2 of PPL in horses was approximately 2 h.