Osteoarthritis (OA) was induced in the rat stifle joint by partial medial meniscectomy (PMM) and transection of the cranial cruciate ligament (CCL). At 10 weeks after destabilization, joint morphologic and pathologic changes were observed, scored, and compared. The intact rat stifle joint was observed in a mid-saggital plane. Articular cartilage of the distal portion of the femur and proximal portion of the tibia had thicker and thinner sites, and the thicker sites were located caudally on the distal portion of the femur and centrally on the proximal portion of the tibia. The two separate triangular portions of the medial meniscus observed in the mid-saggital plane contained a center of ossification in the cranial portion and fibrocartilage in the caudal portion. The synovium was one to three cells thick, and contained rare inflammatory cells. Although lesions were more severe in stifles after PMM, both treatments produced OA lesions that closely simulated OA lesions of other species. Lesions consistent with idiopathic OA included chondrocytic clones with increased metachromasia around them, chondrocytic death, loss of metachromasia, fibrillation, fissuring, erosion of articular cartilage, osteophyte formation, and variable synovial inflammation. The results indicate that PMM and CCL transection in the rat are useful in vivo models for study of the etiopathogenesis of OA and therapeutic efficacy of anti-arthritic drugs and treatment concepts.