The circadian rhythms of the calcium oxalate, calcium phosphate, and brushite saturation levels as estimated by the AP(CaOx), AP(CaP), and AP(Bru) indices (Tiselius), respectively, were studied in 5 healthy males on three different occasions. These indices were calculated from the data of urinary specimens collected in 2.5-hour fractions except during sleep. The calcium oxalate and brushite saturation levels peaked between 5:30 and 8:00 am, and these were significantly higher than in the other periods. The calcium phosphate saturation level had two peaks that occurred between 8:00 and 10:30 am and between 1:00 and 6:00 pm. The ion-activity products of octacalcium phosphate and hydroxyapatite, calculated from the AP(CaP) index, exhibited a pattern similar to the AP(CaP) values, indicating a high risk of crystallization for both substances at around the same peak periods. In conclusion, early morning was found to be the high risk period for calcium oxalate and brushite crystallization, while two high risk periods for octacalcium phosphate and hydroxyapatite were detected between 8:00 and 10:30 am and between 1:00 and 6:00 pm.