Little information is available about the distribution of magnesium in the human bone, although several studies have been reported on the whole sample of bones. Therefore the present study on magnesium distribution in human ribs was carried out. Sixty human ribs obtained from subjects aged 10 to 80 years were cut transversely. Two transverse sections from periosteal to endosteal surface were ground to a thickness of about 1000μm. One section was used for magnesium determination by atomic absorption spectrophotometry and the other for analysis with XMA. Specimens were taken across the rib cortex. Layers 30μm thick were abraded serially from the periosteal to endosteal surfaces by abrasive microsampling as previously described (Weatherell et al. Caries Res. 19; 97-102, 1985). Analysis showed that magnesium concentrations were higher both in periosteal and endosteal surfaces. Neither the profile nor the overall concentration of magnesium, however, changed with age.