Six normal, healthy adult males received a single dose of allopurinol intravenously, orally in the form of a commercial tablet, and rectally in the form of an extemperaneously prepared suppository (either in a cocoa butter or in polyethylene glycol base). Plasma allopurinol and oxipurinol concentrations were measured over a period of at least 60 h. The following mean (+/- SD) values were obtained from the intravenous allopurinol experiment: clearance, 9.62 +/- 3.49 ml . kg-1 . min-1; Vd, 1.61 +/- 0.74 l/kg; t1/2, 1.62 h. Oxipurinol had a mean t1/2 of 16.90 h. The absolute systemic bioavailability of the oral tablet was 67% +/- 23%, while the allopurinol rectal suppositories produced no measurable plasma concentrations of allopurinol or oxipurinol in any of the subjects. Current use of rectal dosage forms as an adjunct in cancer chemotherapy should therefore be re-examined.