Alendronate (4-amino-1-hydroxybutylidene bisphosphonate, ALN) is an aminobisphosphonate that is being developed for the treatment of diseases characterized by increased bone resorption. The purpose of this study was to assess the long-term safety of ALN with special emphasis on bone strength and bone morphology. Thirty-two (16 males and 16 females) 83- to 86-week-old beagle dogs were treated p.o. for up to 3 years with ALN at 0.00, 0.25, 0.5 or 1.0 mg/kg/day. The following parameters of toxicity were assessed: physical signs, body weight, ophthalmology, radiographic evaluation of bone, electrocardiography, hematology, clinical chemistry, urinalysis, necropsy including organ weight assessment, histopathology and biomechanical testing of bone. There were no apparent compound-related alterations in any of the above mentioned parameters except the expected changes (related to the pharmacological activity of ALN) in serum phosphorus and Ca concentrations and in the histology of bones with active endochondral bone formation (rib). There were mild transient reductions in the serum phosphorus and Ca concentrations at the 1.0 mg/kg/day dose level during the early part of the study. There was a dose-dependent delay in bone remodeling in the ribs of all dogs treated with ALN. There was no similar change in the tibia. Most importantly, there were no spontaneous fractures and there were no changes in the structural properties of femoral or vertebral bone. The total ALN content of bone in an average dog (10 kg) after 3 years of treatment with approximately five items of the intended dose for the treatment of osteoporosis was approximately 8 mg, which is only 0.001% of total bone mass (700 g). Introduction.