The renal blood clearance of sodium pentaborate was observed in 8 patients undergoing neutron capture therapy for intracranial malignant tumors and was found to average 39.1 ml./min. per 1.73 sq. m. surface area. The renal blood clearance was calculated for mice receiving sodium pentaborate intravenously in dosages of 25 µ.g./gm. body weight and 100 µg./gm., calculated as boron. The average clearance for the mice receiving the larger dose was 0.207 ml./min. for a 21-gm. mouse. The average clearance for the mice receiving the smaller dose was 0.192 ml./min. per 21-gm. mouse. For the mouse, clearance calculated for a surface area of 1.73 sq. m., as was used in man, averages 40 ml./min. No toxic effects on renal function were observed in mice at the higher dosage.