The relationship between the thickness of articular cartilage and static compressive stress was studied in 6 joints of 5 species of quadruped (mouse, rat, dog, sheep, cow) ranging in weight from 0.05 to 600 Ib. Using free body diagrams and vector analysis, the loads on these joints were determined in a relaxed standing position. Each joint was then loaded to the calculated level by a compression device. A plastic mold of the loaded joint revealed the area of cartilage under compression. For the entire series of animals, maximum cartilage thickness for each joint bore a simple allometric relationship, y = b xk, to body weight, hip-to-shoulder length, and area of the tibial plateau. By contrast, the stresses on the joint cartilages among the various species were generally within one order of magnitude and unrelated to the thickness of the cartilage. Among the species, the low ratio of highest/lowest stress on the femoro-tibial joint suggests that limb structures are optimally designed to maintain low static pressures on the joint surface. These findings do not exclude the possibility that other stresses, noncompressive or dynamic, may be related to cartilage thickness.