The experimental antineoplastic agent temozolomide was not metabolised in vitro at a measurable rate by mouse liver fractions. In contrast, the temozolomide analogue 3-methylbenzotriazinone was metabolically N-demethylated by hepatic microsomes to yield benzotriazinone. The major route of excretion of [14C]-labelled temozolomide in mice was via the kidneys. An acidic metabolite of temozolomide, probably a conjugate, was found in the urine of mice, but its identity could not be established unambiguously. Spectroscopic analysis and chemical tests revealed that it possesses an intact NNN-linkage. Another metabolite was found in the urine of patients but not of mice. This metabolite was identified as the 8-carboxylic acid derivative of temozolomide. Unlike the unknown species, this metabolite was cytotoxic against TLX5 lymphoma cells in vitro.