We studied the effect of variations of fluid intake on the oxalate (Ox) excretion in healthy subjects. The relation of Ox excretion to urine volume was examined. The Ca excretion was measured as well and the ionized Ca and the Ca·Ox product were calculated. With a fluid intake between 800 and 4,200 ml a urine volume between 550 and 4,200 ml was found. We saw a strong correlation between urine volume and Ox excretion and a strong negative correlation between urine volume and urinary Ox concentration. The upper limit of urinary Ox excretion to define hyperoxaluria should therefore be modified by taking the urine volume into consideration or should be replaced by the Ox concentration