The rate and extent of allopurinol absorption was studied following its oral ingestion in a "high frequency capsule" which allows the evaluation of the sites of drug absorption. If allopurinol is liberated in the duodenum or upper jejunum its absorption is fast and complete while its liberation in the lower jejunum results in a slow and incomplete absorption. A capacity limited absorption process for allopurinol, suggested from the results of a study on the allopurinol bioavailability from different formulations, could not be proved in the range of single doses between 200 and 600 mg resp. 2.2 to 12.8 mg/kg. AUC- and Cp-values of allopurinol and oxipurinol correspond to the calculated figures in relation to the different doses/kg.