SummarySequential blood sampling every 30 min was performed in six adolescent males. Samples were analyzed for serum phosphate ((Pi), total calcium (CaT), and blood ionized calcium (Ca++) concentrations. A circadian (24-h) pattern was observed for each mineral: 1) Ca++concentrations followed a U-shaped curve with a peak at 11 AM and a trough at 3:30 PM; 2) CaTconcentrations followed a W-shaped curve with maxima at 11 and 1 AM and troughs at 5 AM and 6 PM; and 3) Piconcentrations followed an M-shaped curve with peaks at 4 PM and 3:30 AM. The overall mineral patterns were similar in shape to those previously decribed in adult males. In contrast, however, the amplitude of the Pifluctuations were considerably greater in the adolescents compared with that in adults (3.0 mg/dl versus 1.2 mg/dl). The Ca++pattern diverged from the CaTpattern during the nocturnal phase to an extent that neither pattern could be predicted from the other. Ultradian rhythms were found in some of the time series mineral data sets; however, no single frequency encompassed all subjects for each circulating mineral.