Background and Aims: The 2-(4-amino-3-methylphenyl)-5-fluorobenzothiazole prodrug Phortress exerts potent and selective antitumour activity in vitro and in vivo. Preclinical toxicokinetic studies in 2 rodent species were undertaken to determine Phortress' maximum tolerated dose and advise a safe starting dose for clinical evaluation. Methods: Plasma pharmacokinetic parameters were determined by high-performance liquid chromatography and fluorescence detection following Phortress administration to mice (10 mg/kg, intravenously on days 1 and 8). Phortress (20 mg/kg, on days 1 and 8) was administered to CYP1A1/ßGAL reporter mice; tissues were examined macro- and microscopically. Toxicological and pharmacodynamic endpoints were examined in organs of rodents receiving Phortress (10 mg/kg or 20 mg/kg, on days 1 and 8). CYP1A1 expression and Phortress-derived DNA adducts were determined in lungs and livers (on days 11 and 36). Results: No accumulation of Phortress was detected in murine plasma. ß-Galactosidase activity inferred Phortress-derived induction of cyp1a1 transcription in the livers of transgenic mice; no total body weight loss was encountered in these animals. However, a fall in lung:body weight and kidney:body weight ratios, raised serum alkaline phosphatase levels and hepatic histopathological disturbances in animals receiving 20 mg/kg Phortress indicate organ sites of potential toxicity. CYP1A1 protein was induced transiently in the lungs of both species and in the livers of rats. Elimination of hepatic DNA adducts and rat pulmonary adducts was evident; however, murine pulmonary adducts persisted. Conclusion: Rodent preclinical toxicology established that mice represent the more sensitive rodent species, resolving a maximum tolerated dose of 10 mg/kg Phortress.