The effect of PTH on chondrocyte proliferation as a function of cartilage age was examined. PTH[1-34] induced a 12- to 15-fold increase in the efficiency of colony formation in soft agar by chondrocytes from embryonic 13- to 19-d-old chickens and fetal 25-d-old rabbits with a 10-fold increase in their DNA content. It also caused a 2.5-fold increase in [3H]thymidine incorporation into DNA in fetal 25-d-old rabbit chondrocytes. No mitogenic responses to PTH were observed, however, in postnatal 7- to 21-d-old chick chondrocytes or postnatal 21-d-old rabbit chondrocytes. This age dependency was observed only with PTH: fibroblast growth factor, epidermal growth factor, and insulin stimulated chondrocyte proliferation irrespective of cartilage age. The absence of a mitogenic effect in postnatal chondrocytes was not due to a decrease in number or a reduction in affinity of receptors for PTH. PTH also increased [35S]sulfate incorporation into proteoglycans and the cyclic AMP level in fetal and postnatal chondrocytes, but at 100-fold higher concentrations (10(-8)-10(-7) M) than those (10(-10)-10(-9) M) required for the stimulation of cell division. These results suggest that PTH is a potent mitogen for embryonic chondrocytes, and that its mitogenic effect disappears selectively after birth.