The effect of zinc on in vitro deoxyribonucleic acid (DNA) synthesis activity in the femoral-diaphyseal and metaphyseal tissues of newborn rats was investigated to determine a role of zinc in bone growth. In vitro DNA synthesis was assayed in a reaction mixture containing the 100 g centrifugation supernatant, which includes the nucleus of bone cells, of bone issue homogenate with incorporation of [3H]-deoxythymidine 5?-triphosphate (dTTP). DNA synthesis activity in the femoral-diaphyseal and metaphyseal tissues of newborn rats was significantly raised with increasing age (1-21 days) after birth. The presence of dipicolinate (10?3 M), a chelator of zinc, in the reaction mixture caused a significant decrease in DNA synthesis activity in the diaphyseal and metaphyseal tissues of newborn rats at 7 and 14 days after birth. The addition of zinc sulfate (10?6 - 10?4 M) resulted in a significant increase in DNA synthesis activity in the diaphyseal and metaphyseal tissues. When the diaphyseal and metaphyseal tissues of newborn rats at 7 days after birth were cultured for 24 hours in a serum-free medium containing either vehicle, zinc sulfate (10?4 M), insulin-like growth factor-I (IGF-I; 10?8 M) or transforming growth factor-b (TGF-b; 10?10 M), bone DNA synthesis activity was significantly elevated. Culture with both zinc and IGF-I enhanced additively bone DNA synthesis activity. Such an effect was not seen in the case of zinc and TGF-b. The effect of zinc, IGF-I, or zinc plus IGF-I in increasing bone DNA synthesis activity was completely prevented by culture with PD98059 (10?5 M), an inhibitor of mitogen-activated protein (MAP) kinase. Also, the effect of zinc, TGF-b, or zinc plus TGF-b in elevating bone DNA synthesis activity was significantly inhibited by culture with staurosporine (10?6 M), an inhibitor of protein kinase C. The present study demonstrates that zinc, like bone growth factors, has a stimulatory effect on bone DNA synthesis in newborn rats.