1. The metabolism of allopurinol was studied in a newly discovered patient with xanthinuria.
2. No oxipurinol was detectable in the urine during allopurinol administration, in direct contrast to the results in a patient previously studied by one of us.
3. Other equally discrepant effects of allopurinol on urinary purine and pyrimidine excretion in these two cases are compared with those in the three other recorded cases.