The route of formation of 1-methyluric acid (1MU) from theophylline has been investigated by i.v. administration of 1-methylxanthine (1MX) before and after allopurinol treatment and of 1,3-dimethyluric acid (1,3DMU) to healthy male volunteers. 1,3DMU was recovered unchanged in urine and was not demethylated to 1MU. By contrast after administration of 1MX the major part of the dose was recovered as 1MU. After allopurinol this was reversed with most of the 1MX dose being excreted unchanged and only a minor amount as 1MU. The results show that 1MU derived from theophylline is formed by an initial 3-demethylation to 1MX and then a rapid xanthine oxidase mediated 8-oxidation. No 1MU is formed by 3-demethylation of 1,3DMU.