A pilot dietary experiment was conducted over 10 days to evaluate whether a simple yet often underutilized approach of constructing mass balance of arsenic metabolites can be used to assess in vivo bioaccessibility of arsenic in cooked rice. Two volunteers were involved in this study. The quantity of drinking water, food and urine samples, together with arsenic concentration and speciation of these samples was monitored to construct a mass balance of arsenic intake and excretion. In the first five days, the two volunteers on a wheat diet had an average arsenic daily intake of 15.4+/-2.6microg and 9.6+/-0.7microg, respectively. In the next five days, these volunteers switched to a rice diet, increasing the average arsenic daily intake to 36.4+/-2.8microg and 34.1+/-7.7microg, respectively. Daily excretion of urinary arsenic, mostly as dimethylarsenic acid (DMA), doubled from 9.8+/-0.3microg to 21.0+/-3.0microg, and from 6.5+/-0.8microg to 11.6+/-4.5microg, respectively. The percentage of ingested arsenic excreted in urine remained constant at approximately 58% for one volunteer before and after the rice diet, and was approximately 69% for another. Mass balance established during a controlled dietary experiment over 10 days is shown to be a useful approach to evaluate in vivo bioaccessibility and metabolism of arsenic uptake from diet and is applicable to study with more subjects.