In our preliminary report we demonstrated PTH receptors on rabbit costal chondrocytes in culture. In the present study regulation of expression of PTH receptors of the chondrocytes by various growth and differentiation factors and its relation to the synthesis of glycosaminoglycan (GAG), a differentiated phenotype of chondrocytes, were investigated. Treatment with retinoic acid decreased GAG synthesis in cultured chondrocytes and the number of PTH receptors, measured by binding of [125I]-[Nle8,18,Tyr34]bovine PTH-(1-34) amide to the cells. However, the affinity of the receptors did not change. The decrease in the number of PTH receptors was dose and time dependent and parallel to the decrease in GAG synthesis. When chondrocytes that had been treated with retinoic acid were cultured in the absence of retinoic acid for 3 days, both their GAG synthesis and their number of PTH receptors were restored. Epidermal growth factor and fibroblast growth factor also decreased both the number of PTH receptors and GAG synthesis in the cells. However, these treatments did not change their affinity. Treatment with insulin-like growth factor-I, (Bu)2cAMP, and transforming growth factor-beta resulted in increases in GAG synthesis as well as in the number of PTH receptors without any change in their affinity. In addition, the PTH-stimulated cAMP level in chondrocytes pretreated with retinoic acid, epidermal growth factor, and fibroblast growth factor was lower than that in control cells. On the other hand, the PTH-stimulated cAMP level in chondrocytes pretreated with insulin-like growth factor-I and transforming growth factor-beta was higher than that in control cells. These observations suggest that the increase in the number of PTH receptors on chondrocytes is closely related to expression of the differentiated phenotype of chondrocytes and that the number of the receptors is a good marker of the differentiated phenotype of chondrocytes.