B-10-Enriched borocaptate (BS11) was administered intraperitoneally to SCCVII tumor-bearing C3H/He mice, Electroporation (EP) was conducted by using a tweezers-type electrode. The B-10 contents in tumors were measured by prompt gamma-ray spectrometry. The colony formation assay was applied to investigate the antitumor effects of boron neutron capture therapy (BNCT) and thereby to estimate the intratumor localization of BS11. The B-10 concentrations in tumors decreased with time following BSH administration, falling to 5.4(+/-0.1) ppm at 3 h, whereas EP treatment (3 repetitions) 15 min after BSH injection delayed the clearance of BSH from tumors, and the B-10 level remained at. 19.4(+/-0.9) ppm at 3 h, The effect of BNCT increased with the B-10 concentration in tumors, and the combination with EP showed a remarkably large cell killing effect even at 3 h after BSH injection. The effect of BNCT, i.e., slope coefficient of the cell survival curve of tumors, without EP was proportional to tumor B-10 level (r=0.982), and that of BSH-BNCT combined with EP lay close to the same correlation line. However, tumors subjected to EP after BSH injection did not show high radiosensitivity when irradiated after conversion to a single cell suspension by enzymatic digestion. This indicates that the increase of the BNCT effect by EP was a consequence of enclosure of BSH in the interstitial space of tumor tissue and not within tumor cells. This is different from a previous in vitro study. The combination of EP and BNCT may be clinically useful, if a procedure to limit EP to the tumor region becomes available or if an alternative similar method is employed.