1. The transport of 6-thioguanine and 6-mercaptopurine has been studied with isolated jejunal loops of mouse small intestine. H.p.l.c. was used to identify and quantify the thiopurines and their metabolites in the serosal secretions. 2. When the lumen of the intestinal loops contained either 6-thioguanine or 6-mercaptopurine at a concentration of 1 mmol/l, the concentration of unmetabolized drug in the serosal secretions reached a maximum of 0.13 +/- 0.02 mmol/l (mean +/- SEM). 3. Analysis of the serosal secretions from the perfusions with either of the drugs revealed the appearance of an unknown compound which had the characteristics of a thiopurine and the same time course of appearance as the unmetabolized drug. Thus 6-thioguanine and 6-mercaptopurine are significantly metabolized during absorption in mouse intestine. 4. The unknown compound was identified as 6-thiouric acid, and with 1 mmol/l 6-thioguanine or 6-mercaptopurine in the lumen the concentration of this metabolite in the serosal secretions rose to a maximum of 0.13 +/- 0.01 and 0.18 +/- 0.03 mmol/l, respectively. At luminal drug concentrations of 0.1 mmol/l, the metabolite accounted for approximately 90% of the serosal thiopurine. 5. After an initial lag period of 20 min, linear rates of appearance in the serosal secretions were obtained for both the unmetabolized drugs and 6-thiouric acid. 6. Addition of the xanthine oxidase inhibitor oxypurinol at a luminal concentration of 0.3 mmol/l prevented the formation of 6-thiouric acid from 6-thioguanine. However, the inhibitor reduced the rate of 6-thioguanine appearance in the serosal secretions by 50%. 7. The conversion of 6-mercaptopurine to 6-thiouric acid was prevented when allopurinol or oxypurinol were added to the lumen. At a luminal drug concentration of 1 mmol/l, allopurinol increased the rate at which 6-mercaptopurine appeared in the serosal secretions by 90% compared with an increase of only 50% with oxypurinol. 8. The transport of water and glucose by the mouse intestinal loops was unaffected by 6-thioguanine or the xanthine oxidase inhibitors. However, 6-mercaptopurine caused significant reductions in the rate of water transport (30%) and glucose transport (39%). These effects were observed at a luminal drug concentration of 0.1 mmol/l and there was no further increase at a drug concentration of 1 mmol/l.