BAPN (0.1 mg/day) was injected into chick embryos for 4 days starting on the 7th day of incubation. On the 11th day, the embryos were administered either 3H-proline or 3H-lysine. 36 h later, the incorporation of each isotope by the periosteal osteogenic cells as well as into bone matrix was investigated by autoradiography. The incorporation of the two isotopes into whole bones was assessed by liquid scintillation counting. 3H-proline incorporation into the cellular or matrical compartments was unaffected by treatment. As compared to the controls, 3H-lysine label in BAPN-treated embryonic bones was significantly higher in the cellular compartment but was reduced over the bone matrix. The data provide the first direct morphological evidence that BAPN probably induces certain changes in the maturation of collagen involving lysyl residues which result in an inhibition of cross-linkage formation in collagen.