In order to avoid other antibiotics which interfere with plasma and CSF level determination of a given antibiotic the authors use a chemical method of established specificity. 15 patients presenting with purulent meningitis were given a standard daily dose of metampicillin and divided into 3 groups of 5. On days 2, 4 and 6 after onset of disease plasma and CSF specimens were taken at respectively 1, 2 or 4 hours following metampicillin I.V. injection according to patient group. All patients were cured between days 7 and 21 of therapy. Tolerance was good. RESULTS --there was no correlation between dose injected and plasma level of drug,--CSF distribution of metampicillin was good and stayed identical throughout the disease period,--peak CSF drug level was delayed as compared to plasma level, occurring more than 4 hours after I.V. injection of metampicillin.