ABSTRACT This investigation was undertaken to evaluate the therapeutic efficacy and safety of glycosylated undenatured type II collagen (UC-II) alone or in combination with glucosamine HCl and chondroitin sulfate in arthritic dogs. Twenty dogs divided into four groups (n = 5) were daily treated orally for 120 days: group I, placebo; group II, 10 mg UC-II; group III, 2,000 mg glucosamine + 1,600 mg chondroitin; group IV, UC-II (10 mg) + glucosamine (2,000 mg) + chondroitin (1,600 mg), followed by a 30-day withdrawal period. On a monthly basis, dogs were examined for overall pain, pain upon limb manipulation, and exercise-associated lameness. Serum samples were analyzed for markers of liver function (ALT and bilirubin) and renal function (BUN and creatinine). Body weight was also measured at a monthly interval. Dogs in group I exhibited no change in arthritic conditions. Dogs receiving UC-II alone showed significant reductions in overall pain within 30 days (33%) and pain upon limb manipulation and exercise-associated lameness after 60 days (66% and 44%, respectively) of treatment. Maximum reductions in pain were noted after 120 days of treatment (overall pain reduction, 62%; pain reduction upon limb manipulation, 91%; and reduction in exercise-associated lameness, 78%). The overall activity of the dogs in the UC-II supplemented with glucosamine and chondroitin group (group IV) was significantly better than the glucosamine + chondroitin-supplemented group (group III). Glucosamine and chondroitin alleviated some pain, but in combination with UC-II (group IV) provided significant reductions in overall pain (57%), pain upon limb manipulation (53%), and exercise-associated lameness (53%). Following withdrawal of supplements, all dogs (groups II to IV) experienced a relapse of pain. None of the dogs in any groups showed any adverse effects or change in liver or kidney function markers or body weight. Data of this placebo-controlled study demonstrate that daily treatment of arthritic dogs with UC-II alone or in combination with glucosamine and chondroitin markedly alleviates arthritic-associated pain, and these supplements are well tolerated as no side effects were noted.