A new system has been developed for stereotactically delivering prescribed high doses of radiation to precisely located volumes of approximately 0.6 to 10.0 ml within the brain. A Brown-Roberts-Wells stereotactic apparatus and a 6-MeV linear accelerator equipped with a special collimator (12.5 to 30 mm in diameter) have been adapted. The 20-mm collimator allows treatment of a nearly spherical volume of 2.1 ml. Outside the treatment field, the dosage declines to 80% of the dose prescribed for the periphery of the lesion over a distance of 1.8 mm and to 50% over the next 3.4 mm. Localization can be accomplished via computed tomography or cerebral angiography. Treatment is accomplished with an arcing beam of photon radiation with the turntable (couch) in each of four positions. The entire system has been extensively tested for accuracy in alignment and distribution of radiation. Errors have been measured for the alignment of the apparatus and for the process of localization. Safety of operation was emphasized throughout the design and testing phase.