The influence of the administration schedule (intravenous (i.v.) bolus versus i.v. infusion) on the pharmacokinetics of methotrexate (MTX) in plasma and extracellular fluid (ECF) of a brain C6-glioma was investigated in rats. MTX concentrations were determined by high performance liquid chromatography (HPLC)-ultraviolet radiation (UV). MTX (50 mg/kg) was administered by i.v. bolus or i.v. infusion (4 h). Concentration-time profiles were fitted to a two-compartment open model. Maximum MTX concentrations ranged between 178 and 294 microgram/ml (i.v. bolus), and between 11 and 24 microgram/ml (i.v. infusion) in plasma. MTX rapidly entered the tumour tissue although its concentrations in the ECF were much lower than those observed in plasma for both modes of administration. In spite of an important interindividual variability, AUC(ECF) was approximately 5-fold higher and mean MTX penetration in tumour ECF (AUC(ECF)/AUC(Plasma)) was approximately 3-fold higher after i.v. bolus than after i.v. infusion administration. These results indicate that i.v. bolus administration schedules promote MTX delivery in brain tumour tissue.