In three groups--patients with recurrent calcium urolithiasis (RCU), patients with primary hyperparathyroidism (pHPT), and healthy controls--citrate was measured enzymatically in 24 and in 2-hr urine after an overnight fast. Citrate excretion per 24 hr was significantly lower in RCU than in age and sex matched controls, whereas there was no significant difference in citrate excretion in urines from the 2-hr morning collection. In pHPT citrate was also lower than in controls and fell within the range of RCU of comparable age. Both categories of urines (24 and 2hr) have in common the characteristic that the actual citrate concentration is lower by 50 per cent in RCU and pHPT than in controls, mainly as a result of the higher urine volume. Correction of citrate for creatinine does not disclose further differences among the populations studied but conversely hampers exact interpretation of urinary citrate in the absence of strict separation of individuals according to sex and age. From these data we conclude that (i) a low excretion and concentration of urinary citrate is detectable in calcium lithiasis and may contribute to a deficiency in inhibitory activity against nucleating processes in stone-forming urine; and (ii) the differences in urinary citrate elicited in samples of 24 and 2-hr morning urine are of unknown origin and merit further investigations.