The local tissue metabolism is controlled through the complex interaction between systemic and local growth factors. In recent years, an increasing number of autocrine or paracrine growth regulators have been identified in physeal cartilage. While these factors act to alter chondrocytes phenotypically and presumably are important mediators in the process of endochondral ossification, the manner in which they interact with the systemically regulated growth factor insulin‐like growth factor‐I is unknown. In the present study, the interactive effects of insulin‐like growth factor‐I with transforming growth factor‐β1 or basic fibroblast growth factor were examined in short‐term monolayer cultures of chick growth plate chondrocytes. [3H]thymidine incorporation was maximally stimulated 11‐fold by fibroblast growth factor (10 ng/ml) and 3.5‐fold by transforming growth factor‐β1 following a 24‐hour exposure in serum‐containing cultures. The effects of transforming growth factor‐β1 and fibroblast growth factor at both high and low concentrations were enhanced in a dose‐dependent manner by insulin‐like growth factor‐I, with a 40–50% increase in DNA synthesis in the presence of 100 ng/ml of insulin‐like growth factor‐I. Since insulin‐like growth factor‐I increased [3H]thymidine incorporation after 48 hours (50% increase) but not after 24 hours of exposure, these observations represent a synergistic interaction. Total DNA in cultures treated for 5 days confirmed the modulating effect of insulin‐like growth factor‐I with transforming growth factor‐β1 and fibroblast growth factor. The growth factors were further examined for their effects on markers of chondrocyte differentiation. While all three caused a dose‐dependent inhibition of alkaline phosphatase activity, the effects of insulin‐like growth factor‐I were additive only to those of transforming growth factor‐β1 and fibroblast growth factor. Similarly, insulin‐like growth factor‐I did not affect the sulfate incorporation stimulated by fibroblast growth factor or transforming growth factor‐β1. Insulin‐like growth factor‐I had no effect on total protein synthesis after 24 hours and, although type‐II collagen mRNA levels were stimulated, it had no effect on type‐X collagen mRNA, as determined by quantitative in situ hybridization. Finally, insulin‐like growth factor‐I did not alter the dose‐dependent stimulation of noncollagen protein synthesis and the inhibition of collagen synthesis caused by fibroblast growth factor and transforming growth factor‐β1 in 24‐hour cultures. Thus, the data suggest that insulin‐like growth factor‐I may have a role in augmenting the effects of other growth factors found in cartilage. Since insulin‐like growth factor‐I is under systemic control by growth hormones, this permits an endocrine regulation of transforming growth factor‐β1 and fibroblast growth factor activity and may bring local growth factor effects under systemic control.