Oxipurinol plasma levels and plasma elimination half-life were investigated in five healthy volunteers after oral administration of 300 mg allopurinol in customary (A 300) and in slow-release preparation (A ret) in a double blind cross-over study. After a single oral dose of 300 mg allopurinol in customary preparation maximum oxipurinol plasma levels were 5.24 microgram/ml. After allopurinol in slow-release preparation maximum oxipurinol levels were 2.22 microgram/ml. 24-h oxipurinol plasma levels were 3.78 microgram/ml after A 300 and 2.08 microgram/ml after A ret, respectively. Chronic oral administration of a daily dose of 300 mg allopurinol resulted in accumulation of oxipurinol until day 8. 24-h steady-state levels of oxipurinol were found to be 9.98 microgram/ml after A 300 and 7.14 microgram/ml after A ret. All oxipurinol plasma levels on A 300 exceeded those on A ret. The time course of oxipurinol plasma levels after a single oral dose of allopurinol suggests that oxipurinol elimination from plasma follows an exponential function (first-order elimination). Plasma elimination half-life of oxipurinol was 42.65 h after administration of A 300 and 49.91 h after administration of A ret. The difference between the elimination half-lives is statistically not significant.