The penetration of antibiotics into the cerebrospinal fluid (CSF) is governed by several factors, including lipid solubility, degree of ionization, pH gradient, serum protein binding, molecular weight and structure and, above all, degree of meningeal inflammation. The distribution within, and elimination from the subarachnoid spaces depend upon the physiology of the CSF and the route of administration of the drug. CSF penetration studies carried out in patients or volunteers with various groups of antibiotics and antibacterial agents have shown that, except for chloramphenicol and co-trimixazole, passage through the blood-brain barrier is generally poor. However, when the meninges are inflamed therapeutically active concentrations of penicillins and of some cephalosporins can be measured in the CSF.