A male non-smoking volunteer increased his daily intake of ascorbic acid continuously by ingesting in a single, oral dose 1, 2, 3, 4 and 5 g crystalline ascorbic acid. The dose was always kept constant for 2 days and was taken with breakfast. During the regimen of ascorbic acid ingestion blood samples were taken and all urine was collected. Then, with a 5 g dose of ascorbic acid 8.17 micro Ci (170 microgram) (1-14C)ascorbic acid was ingested. Following the labelled dose radioactivity was determined in plasma, urine, and faeces. Daily ingestion of 5 g ascorbic acid was continued for 10 days. During the whole experiment the ascorbic acid concentrations in plasma and in urine were determined. The urinary excretion of unmetabolized unlabelled ascorbic acid per day was taken as index for the absorption of ascorbic acid. It decreased from 75% (1 g), 44.0% (2 g), 39%, (3g), 28% (4 g) to 20% (5 g) of the ingested ascorbic acid. The linearisation of the relationship between ingested dose and the excreted amount in urine yielded a maximum amount of 1160 mg ascorbic acid which can be absorbed in 24 h under the conditions of the experiment. The experiment with (1-14C)ascorbic acid revealed that a single oral dose of 5 g ascorbic acid is absorbed to 22.2%, in the faeces collected up to 117 h following ingestion of label only 2.8% of the radioactivity could be recovered. Several possibilities are discussed regarding the fate of the remaining unabsorbed (1-14C)ascorbic acid.