The effect of fasting and refeeding on the uptake and retention of purines by the small intestine of the rat was studied in vivo. Short-term uptake and incorporation into nucleotides of the purine bases adenine, guanine and hypoxanthine and the nucleoside inosine were evaluated in the proximal jejunum. After 5 min, more label was recovered in the intestinal contents in fasted rats, indicating that total absorption was reduced. However, intestinal retention of purines (50 nmol dose) was elevated with fasting (27.2 vs. 16.6 nmol/g for adenine, 5.7 vs. 3.0 nmol/g for guanine and 16.1 vs. 7.4 nmol/g for hypoxanthine, for fed vs. fasted, respectively). After 1 day of refeeding, retention remained elevated for adenine (27.4 nmol/g) and guanine (5.5 nmol/g). After 3 days of refeeding intestinal weight and retention of labeled purines returned to the unfasted levels. Nucleotide formation from all purine bases was greater in the intestinal tissue of fasted as compared to fed rats (25.4 vs. 11.4 nmol/g for adenine, 1.32 vs. 0.24 nmol/g for guanine, and 2.84 vs. 0.82 nmol/g for hypoxanthine). At a higher dose (3000 nmol) hypoxanthine and inosine were retained to a greater extent in the fasted than in the fed state. Pretreatment with allopurinol (a xanthine oxidase inhibitor) reduced the absorption of hypoxanthine, increased the retention of label in the tissue 4-fold or more, and elevated nucleotide formation 10-fold or more. Fasting and allopurinol treatment, both known affectors of xanthine oxidase activity, enhanced both the retention of dietary purine and nucleotide formation.