Allopurinol is converted to a presently unidentified metabolite by whole blood [human], in samples prepared in vitro and samples from subjects administered the drug. The apparent plasma concentration of allopurinol, as determined by HPLC [high pressure liquid chromatography] is dependent upon length of time between sample preparation or sample withdrawal and the centrifugation and removal of plasma. Oxypurinol does not undergo the same conversion, nor does it affect loss of allopurinol.