p38 mitogen-activated protein kinase inhibition blunts transforming growth factor beta (TGF-β)-stimulated, but not insulin-like growth factor 1 (IGF-1)-stimulated, proteoglycan synthesis. Rabbit chondrocytes were grown to confluence, the medium serum reduced, and 1 μM SB 203580, and 50 ng/ml IGF-1 or 50-pM TGF-β added. The cells were pulse labeled 24 hours later (for 6 hours) with 35S-sulfate. Conditioned media and cell extracts were assayed for incorporation of label into proteoglycans by chromatography on PD-10 columns, and the data are expressed as pmol/105 cells. Values are the mean ± standard error of n = 6–12. *P < 0.05 versus vehicle. SB, SB 203580.