The temporal relationships between the circadian rhythms of serum PTH, total calcium (Cat), and phosphate (Pi) and plasma ionized calcium (Cai) concentrations were determined in 9 normal men. Blood samples were collected every half hour for 24 h. Serum PTH was measured by an RIA specific for the midregion of the molecule. The mean circadian pattern for each variable was derived by calculating the average value across all men at concurrent time points. After the data were smoothed by the method of running means, the correlations between PTH and mineral values from concurrent time points were calculated, as were cross-correlations to 12 lag periods (6 h). Spectral and cross-spectral analyses were performed on the same data set. Both statistical methods yielded consistent results: 1) at concurrent time points (0 lag), high correlations were found between serum PTH and Cat (r = -0.74), PTH and Pi (r = 0.79), and PTH and Cai (r = -0.53); and 2) when the PTH series was lagged by 2 h, the PTH/Cai correlation improved to -0.70. A direct PTH/Cai correlation of 0.50 was found when the Cai series was lagged about 4.5 h. No improvement in the correlations were found when the other series were lagged. Spectral analyses also detected significant interrelations between PTH and Cai at 2 and 3.5 h. These data describe the timing of the bidirectional interaction between serum PTH and plasma Cai under steady state conditions in normal adult men; changes in Cai concentrations precede inverse changes in PTH levels by 2 h, whereas changes in PTH precede similar directional alterations in Cai by about 4 h.