The concentration of 2-sulphanilamido-5-methyl-pirimidine (SMP) was studied in the CSF and in the blood of 30 patients with diseases of the nervous system. Two groups of patients were considered. The first group was represented by 19 patients in whom the CSF was found to be normal: 10 cases of epilepsy, one case of hydrocephaly, three cases of peripheric neuropathy and five cases of chronic impairment of the spinal cord associated to gastricochloridria. Among the last patients, there was a diminution of the ab-sortion of vitamina B12 in two, as it was evidenced by the radiocyanoco-balamin absorption test (cases 15 and 19). The second group is represented by 11 patients with changes in the CSF composition: 4 patients with polyradiculoneuritis, one patient with neurosyphilis and 5 patients with cysticercosis of the central nervous system. CSF changes were characterized by the occurrence of protein-cytologic dissociation among the first 4 patients and by pleocitosis of the polynuclear type in the case of meningitis. Complement fixation tests for syphilis in the CSF were positive in the case of neurosyphilis and that for cysticercosis in the CSF of the last 5 cases. SMP was administered orally to all patients: 1 g was given initially, and 0.5 g after 12 and 24 hours. CSF and blood samples were collected 36 hours after the first administration. The concentration of sulfanilamide was determined by the method of Bratton and Marshall with a correction for SMP. The results are presented in the table 1. Table 2 shows the mean values obtained in the first group of patients. A positive correlation was found between the concentration of SMP in the blood and in the CSF among the cases of this group. The value of the correlation was statistically significant. The quotient between the concentration of SMP in the CSF and in the blood was represented by 0.203 ± 0.087. The blood concentration of SMP was found to be the lowest in the two cases with deficient absorption of radiocyanocobalamin. The results obtained for the first group of patients were used as a control for the results obtained for the second group of patients. The results were found to be similar in both groups of patients, with exception of two cases of polyradiculoneuritis. In these the quotient of CSF and blood titer of SMP was found to be increased, due to a relative higher titer in the CSF than in the blood. This is interpreted as due to deficient re-absorption of the CSF, which is admitted to occur in polyradiculoneuritis.