Methadone hydrochloride is an opiate that has pharmacodynamic and pharmacokinetic properties that suggest it may provide longer analgesia than morphine when administered via the intra-articular route. However, no studies to date have been conducted examining the effects of intra-articular methadone hydrochloride on local tissues. Therefore, the purpose of this study was to determine the histopathologic effects of intra-articular methadone hydrochloride on local tissues in the canine knee. Nine canines, 1 to 4 years old, weighing between 20 kg and 23 kg were used. All canines had their knees randomized to receive either bupivacaine, 0.5% with epinephrine 1:200,000 (4.5 mL), and 5 mg methadone hydrochloride (0.5 mL) for the study knee, or bupivacaine, 0.5% with epinephrine 1:200,000 (4.5 mL), and 0.5 mL normal saline for the control knee. Serum methadone hydrochloride levels were obtained on all canines at 6 and 24 hours. Canines were randomly assigned to 1 of 3 groups to be euthanized at either 24 hours, 14 days, or 28 days. Following euthanization and necropsy, synovial fluid levels and tissue samples were obtained and examined for histopathologic changes. Synovial fluid samples noted a few white blood cells at 24 hours and none at 14 and 28 days. Tissue samples showed no histopathologic changes, and serum concentration levels of methadone hydrochloride were negligible.