Uridine diphosphate, at a concentration of one millimole per milliliter, was found to stimulate the rate of incorporation of 35SO4 and 3H-glycine into articular cartilage both in vivo and in vitro. These findings suggest that uridine diphosphate somehow acts to increase proteoglycan synthesis by the chondrocyte. The mechanism of action is obscure, but the data suggest that uridine diphosphate may act as a stimulator of ribonucleic acid synthesis.