The short-term effects of different intakes of Ca and oxalic acid on excretion of those substances in urine were studied in 8 normal men and 8 with a history of renal stones containing Ca. The effect of oxalate in the diet on oxalate in urine depended in part on intake of Ca. With a normal intake of Ca an increase of oxalate in diet gave an increase in oxalate excretion corresponding to 3.6% of the extra oxalate given whereas on a diet low in Ca the increase in oxalate excretion corresponded to 8.1% of the extra oxalate given.Reducing daily Ca intake from 1000 to 250 mg reduced Ca excretion by 150 mg daily in the patients and 60 mg in the controls, and increased oxalate excretion by 10 and 7 mg daily, respectively, calcium oxalate activity products (Robertson, W.G. et al., Clinical Science (1968) 34, 579) remaining almost unchanged. When Ca and oxalate were reduced in diet there was a fall in excretion of Ca but no rise in that of oxalate, and calcium oxalate activity product fell significantly in patients and controls.