The effect of zinc on bone growth in newborn rats supplied with lactation by maternal rats was investigated. Newborn rats were killed between 1 and 35 days after birth. Increasing age caused a significant increase in zinc content, calcium content, and alkaline phosphatase activity in the femoral-diaphyseal and metaphyseal tissues, while the bone deoxyribonucleic acid (DNA) content was significantly decreased because of elevation of mineral content. Oral administration of zinc sulfate (2.0 mg/100 g body weight; four times at 24-h intervals) to maternal rats from 1 day after birth induced a significant increase in zinc, alkaline phosphatase activity, DNA, and calcium content in the femoral-diaphyseal and metaphyseal tissues of newborn rats compared with those 7 or 14 days old. The results indicate that the increase in bone components results from lactation with zinc-containing milk of maternal rats. The femoral-metaphyseal tissues of newborn rats obtained at 7 days after birth were cultured for 24 h in a medium containing either vehicle or zinc sulfate (10⁻⁶ to 10⁻⁴ M) in vitro. Bone alkaline phosphatase activity and calcium and DNA content were significantly increased by zinc addition. These increases were completely prevented by the presence of dipicolinate (10⁻³ M), a chelator of zinc ion, or cycloheximide (10⁻⁶ M), an inhibitor of protein synthesis. The present study suggests that zinc plays a role in the development of bone growth in newborn rats.