Serum magnesium has been determined by atomic absorption spectrophotometry in a national sample of the United States population between 1971 and 1974. Mean values and percentile distributions are presented for ages 1-74 in males and females in two racial groups. Mean values decreased in both sexes from age 1 to ages 12-24. In females, they increased again after age 25, while males showed little change after age 17. Small sex differences were observed between ages 18 and 45, with men having higher levels than women. Both white males and females had higher serum levels than black males and females of the same age. These differences were statistically significant in many age groups, particularly in young and middle-aged adults. These data are unique inasmuch as they represent the only broad population measurements of serum magnesium in the United States. No comparable data exist as far as is known. The values shown can be considered as normative for the U.S. population. A review of the literature with comparable data was done and results are included; a discussion of the findings from these studies is presented.