The inhibitory activity (IA) of calcium oxalate crystallization was investigated in healthy Indians and renal stone formers. A marked circadian rhythm was observed in men and women of both groups. The rhythm pattern was similar on the basis of sex; however, it was different in stone formers as compared to normals. The acrophase for crystallization of calcium oxalate was observed, respectively, at 05.43 and 05.52 h in healthy men and women, and at 09.52 and 09.23 h in male and female renal stone formers. Moreover, the acrophase of percent inhibition by IA was observed at 17.44 and 17.52 h in healthy participants and at 21.52 and 21.27 h in male and female stone formers, respectively. Therefore, the acrophase suggests a highest risk of crystallization during the morning hours and a minimum one during the evening hours. The day-night rhythmic pattern of urinary calcium, magnesium and phosphorus is discussed in relation to the rhythm of IA.