Volumes and surfaces of articular cartilage and subchondral bone of the proximal tibial epiphysis were measured by unbiased stereological methods on the light microscopic level in groups of 6, 12, and 30-month-old (adult, middle-aged, and old) guinea pigs with primary osteoarthrosis. At 12 months, structural changes similar to those of human osteoarthrosis had developed, predominantly on the central medial condyle, which was not covered with meniscus. The lateral condyle was virtually unaffected; this allowed separate analysis of age-related and disease-related changes. Fibrillation and destruction of cartilage was accompanied by a simultaneous increase of the volume of both cartilage (66%) and subchondral bone (50%). The epiphyseal volume increased by 27% at 12 months, predominantly on the medial (osteoarthrotic) condyle, whereas the volume of the lateral condyle increased only in the oldest age group; this indicated that the joint has a potential for growth and remodeling. Joint growth has been suggested as a pathogenic factor in osteoarthrosis. The 65% increase in thickness noted in subchondral bone was a further indication of the importance of proliferative bone changes in early osteoarthrosis. The low variability of osteoarthrosis in this animal model makes it possible to obtain stable quantitative data from relatively small groups of animals.