In male mice of strain C57Bl, obesity caused by intraperitoneal administration of aurothioglucose failed to alter the course of spontaneous osteoarthrosis. The distribution of osteoarthrosis of the knee joint and of spondylosis of the thoracic and lumbar spine was haphazard in regard to body weight, and levels of plasma and hepatic lipids. It is concluded that the arthrosis-promoting effects of fat-enriched diets are not mediated by the simultaneously occurring obesity.