Hyperoxaluria is a major risk factor for renal stones. In most cases, it is sustained by increased dietary loads. In healthy individuals with a normal Western diet, the majority of urinary oxalate is usually derived from endogenous metabolism. However, up to 50% may be derived from the diet. We were interested in the effect of a high-oxalate diet on oxalate absorption, not merely on the frequently studied increased oxalate excretion. In study I, 25 healthy volunteers were tested with the [13C2]oxalate absorption test once while following a low-oxalate (63 mg) and once while following a high-oxalate (600 mg) diet for 2 days each. In study II, four volunteers repeated study I, and afterwards continued with a high-oxalate diet (600 mg oxalate/day) for 6 weeks. In the last week, the [13C2]oxalate absorption test was repeated. After 4 weeks of individual normal diet, the oxalate absorption test with a high-oxalate diet was performed again. The results of study I show that the mean [13C2]oxalate absorption under low-oxalate diet was 7.9 +/- 4.0%. In the presence of oxalate-rich food, the percent absorption for the soluble labelled oxalate almost doubled (13.7 +/- 6.3%). The results of study II show that the mean [13C2]oxalate absorption of the four volunteers under low-oxalate diet was 7.3 +/- 1.4%. The absorption increased to 14.7+/-5.2% under 600 mg oxalate. After 6 weeks under a high-oxalate diet, the [13C2]oxalate absorption was significantly decreased (8.2 +/- 1.7%). After the wash-out phase, the absorption was again high (14.1 +/- 2.2%) under the 600 mg oxalate challenge.