1. A method for assessing cholecalciferol absorption in man is described. 2. The intestinal absorption of [3H]cholecalciferol was studied in 20 female geriatric patients, most of whom were vitamin D-depleted. 3. The plasma [3H]cholecalciferol response after oral ingestion was significantly lower than that of a group of younger female subjects. 4. The plasma response of labelled polar metabolites of cholecalciferol was also lower in the geriatric than in the younger group, suggesting that increased removal of label by conversion into more polar metabolites could not account for the reduced plasma [3H]cholecalciferol response. 5. There was no evidence that alteration in gastrointestinal motility could account for the different rate of appearance of the labelled vitamin in the plasma in the two groups. 6. It is suggested that there is a defect in intestinal absorption of cholecalciferol in the elderly.