The synthesis of proteoglycans was measured in normal equine articular cartilage of ages 9 months to 20 years and the effect of TGF-beta 1 on this activity was investigated. The rate of incorporation of [S-35]Na2SO4 decreased with age as did the responsiveness of the tissue to the growth factor. The enhanced synthesis of proteoglycan induced at all ages by TGF-beta 1 was down-regulated by IL-1 beta and retinoic acid. The expression of mRNA for TGF-beta 1, 2, and 3 was also measured, and although the level of TGF-beta 1 was highest at all ages, the expression of each growth factor decreased with age.