Volunteers were given a steady intake of various individually different daily dosages of ascorbic acid. After 3 weeks 1-14C-labelled ascorbate was given together with various amounts of unlabelled ascorbic acid (90-1000 mg). Regardless of the total daily dose in cases where the carrier dose amounted to 180 mg or more, carbon dioxide was recovered from the breath. The amount recovered ranged from 1 to more than 30% of the given dose. The larger the amount of carrier the larger was the amount of label recovered as carbon dioxide. It is suggested that the formation of carbon dioxide is due to a presystemic effect as a result of microbiological or chemical degradation of ascorbate in the intestine.