Abstract Changes in serum levels of Ca, Mg, and P during oral glucose tolerance tests (OGTT) in 54 normal children and adolescents (N) and in 69 with abnormality in glucose tolerance were explored. OGTT classification was based on the range and 2 SD distribution of values in the normal population. Borderline tests (B) were those with a single postingestion level above the criterion but less than 200 mg/dl; a chemical diabetes test (C) was one with two abnormal levels or a single postingestion concentration above 200 mg/dl. Subsequent normal tests in youngsters who had previously had an abnormal glucose tolerance test (N 1) were analyzed separately. Mg and Ca declined proportionately in each of the test types. The mean maximal percentage decline in N 1, B, and C tests differed from N means at the p < 0.005 level or less. N 1 tests, despite glucose levels similar to N tests, had <60% the average Mg and Ca decline. At each test time, patient group means for percentage [Ca] and [Mg] decline from baseline were significantly less than N means, with the exception of 4-hr [Mg] mean declines. Mean [P] decline was greater than N in B and C tests at 1 2 hr and in C tests at 3 and 4 hr, mirroring the hyperglycemia. Maximum [P] decline, however, did not vary significantly between normal and N 1, B, or C tests. Ca and Mg concentrations in serum declined with glucose ingestion to a degree similar to known [P] decline. However, abnormal glucose tolerance and normal tests in persons who previously had abnormality were associated with significant diminution of this effect, while [P] change was not similarly affected.