A study of circadian variation in unstimulated submandibular and stimulated parotid saliva has shown that concentrations of calcium and phosphate in saliva show well-defined and opposite variation during the 24 h. Individual variation exists in the amplitude and phasing of these rhythms. This factor must be considered in evaluating results of calcium and phosphate estimations and in the use of figures calculated from these concentrations. Circadian variations may explain some of the discrepancies in previous reports on salivary calcium and phosphate. Since Ca×P products show circadian variation, the composition of the oral environment varies at different times of day. It may be, therefore, that the progress of oral disease varies as a consequence.