The integrity of the blood-brain barrier may be reflected by the blood and cerebrospinal fluid (CSF) if these two compartments are sampled at an appropriate interval after the administration of certain substances. After reading through the controversial literature, this study was undertaken to determine the frequency with which 99mTc-sodium pertechnetate entered the CSF to an abnormal extent, and to see whether this was related to any particular pathology. The plasma-to-CSF ratios were determined in 51 patients 4 hours following the intravenous administration of 0.5-10 mCi 99mTc-sodium pertechnetate. In 23 patients with active CNS tuberculosis, the mean value of the pertechnetate plasma-to-CSF ration was 32.12. In contradistinction, the plasma-to-CSF pertechnetate in 28 nontuberculous subjects was considerably higher (144.63). For the purpose of correlation, a 82Br partition test was also done on each subject 48 hours following the oral administration of NH82Br. The 48-hour study in each of these cases was generally in agreement with the pertechnetate studies: the plasma-to-CSF ratio mean value was 1.25 in patients with active CNS tuberculosis, while for the nontuberculous patients it was 3.32.