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Urinary citrate excretion was estimated colorimetrically from urine samples collected every 3 h for 24 h from 25 healthy adult males (non-stone formers; mean age 39 +/- 7 years) and 25 male patients suffering from calcium nephrolithiasis (stone formers; mean age 41 +/- 6 years). The 24 h citrate excretion was 2.47 +/- 0.65 mmol in non-stone formers and 2.02 +/- 0.71 mmol in stone formers. This difference was not significant. However, cosinor rhythmometry revealed a significant circadian rhythmicity in urinary citrate excretion in the healthy males which was absent in the stone formers; the amplitude was 0.06 mmol in non-stone formers and 0.017 mmol in stone formers. The acrophase was located at 14:25 h in non-stone formers and at 23:30 h in stone formers.