These studies were undertaken in an attempt to demonstrate sensitivity of cartilage from hypophysectomized rats to growth hormone (GH) in physiologic concentrations in vitro and to compare the actions of the hormone and extrinsic insulin-like growth factor-I (IGF-I). In a HEPES-buffered, amino acid-glucose nutrient medium with a low concentration of bovine serum albumin, costal cartilage explants from such animals were responsive to purified or recombinant bovine GH (bGH) and recombinant human GH (hGH) with a sensitivity of less than 100 ng/ml [corrected]. Sulfate incorporation into proteoglycans and thymidine incorporation into DNA were increased. The effect on sulfate incorporation was near maximal by 24 hours, but the peak effect on thymidine incorporation was delayed. Comparison of the dose-response relationships of bGH and IGF-I demonstrated a greater effect of IGF-I with increasing concentrations of each. The action of bGH was inhibited by an immunoglobulin G fraction of an IGF-I antiserum. These results are consistent with previously available evidence that IGF-I has an autocrine/paracrine action on cartilage, which is regulated by GH. However, the limited cartilage response to the direct action of GH supports the hypothesis of Daughaday that IGF-I also has an endocrine role in the stimulation of skeletal growth by the pituitary hormone.