Yearly changes in serum electrolytes, especially calcium (Ca), and Ca intake which may influence such changes were studied for 2 years from 1979 to 1980 in the same inhabitants of a rural area with a tendency to low Ca intake, and the following results were obtained.
1) Among different components of electrolytes, yearly changes in Ca were greater than in magnesium and inorganic phosphorus.
2) Yearly changes in serum Ca level from 9 mg/dl or lower level to higher level or vice versa were more frequent in men than in women and the number of women with not more than 9 mg/dl showed a tendency to decrease.
3) Yearly changes in serum Ca were closely correlated with Ca intake. It was supposed that serum Ca level in those who took about 400 mg/day of Ca was fluctuating above or below 9 mg/dl level in proportion to amount of intake.