The change of lactic acid concentration in the tumour after intraperitoneal administration of 5 g/kg glucose, and the effect of the lactic acid concentration on antitumour activity of hyperthermia were studied in an experimental murine tumour. The lactic acid concentration in SCC VII tumours transplanted into the legs of C3H/HeJ male mice was measured by gas chromatography. Local hyperthermic treatment to the tumour was performed with a water bath at 43 degrees C for 40 min. Antitumour effects were evaluated by measuring tumour volume doubling time (DT) as an index. The mean concentration of lactic acid in the tumour was 10.4 mumol/g in the no-treatment group. The lactic acid concentration gradually increased after glucose administration, reaching a significantly high concentration of 20.0 mumol/g at 90 min later. The DTs in the no-treatment group and hyperthermia alone group were 2.4 and 3.7 days respectively. The DTs in the glucose administration groups shortly before, 30 and 60 min before the hyperthermia were 3.6, 3.6 and 5.6 days respectively. The DT in the 60 min group was significantly extended (p < 0.0001). Hyperthermia during the period of increased lactic acid concentration significantly prolonged the DT of the tumour. These results clearly showed that an increase of lactic acid concentration in the tumour improved the effect of local hyperthermia.