Abstract. We have studied the effect of the somatomedin IGF-I on stimulation of synthesis of DNA, RNA, protein, and glycosaminoglycan in chick chondrocytes. Stimulation of RNA and protein synthesis was rapid, occurring within 10 min after addition of IGF-I. Stimulation of sulphation required a lag time of about 2 h, whether chains were initiated on native protein core or on the artificial acceptor 4-methyl-umbelliferyl-β-D-xyloside. DNA synthesis was not stimulated until after 10 h of exposure to IGF-I. Investigation of the rate of processing of proteoglycan core protein after pulselabelling chondrocytes with 35SO4 yielded data which were best described by a biexponential function. IGF-I had no effect on the t½ of the initial phase (16.44 min in the absence of IGF-I and 20.38 min in the presence of IGF-I). However, addition of IGF-I resulted in a decrease in the t½ of the terminal phase from 122.58 to 55.44 min, which may reflect an increase in synthesis in the enzymes necessary for polymerization and sulphation of proteoglycan.