The route of nutrients going to articular cartilage was studied by determining the diffusion of hydrogen molecules from the subchondral circulation to the articular cartilage in rabbits. In all immature animals there was diffusion of hydrogen from subchondral bone into articular cartilage, while in the older immature animals the results were variable. None of the mature animals showed any diffusion of hydrogen into articular cartilage. The rate of diffusion of hydrogen was significantly lower in the articular cartilage than in the subchondral bone in the immature animals while the concentrations of hydrogen in the articular cartilage were only fractions of those in the subchondral bone at the same instant. Histologically, the deep layers of immature cartilage are penetrated extensively by vascular buds from the ossified portion of the epiphysis, while in adults the articular cartilage is separated from subchondral vascular spaces by an end-plate of bone. Blood vessels penetrating into the basilar layer of articular cartilage in immature animals appear to play an important role in the nutrition of articular cartilage coming from the subchondral region.