Penetration of the aminoglycoside, amikacin, into the cerebrospinal fluid (CSF) of twenty children with acute bacterial meningitis was studied at various times after intramuscular administration and at various stages of therapy. Six of the patients were evaluated during therapy with amikacin at 7.5 mg/kg (intramuscularly) every 12 hours plus ampicillin every 6 hours at 300 mg/kg/day (intravenously); thirteen of the remaining fourteen patients were treated with ampicillin alone, but were given a single intramuscular dose of 7.5 mg/kg of amikacin for evaluation of CSF concentration. Amikacin concentration in CSF with respect to time after administration followed essentially the same pattern as in serum. A minimum concentration of 2 microgram/ml was found in 76% of the CSF samples obtained between 0.5 and 7 hours after administration. A mean amikacin serum/CSF ratio of 3:1 was demonstrated up to 7 hours after dose in all patients who underwent clinical improvement. Patient response was predictable by a correlation of in vitro MIC values with in vivo CSF concentration in three of the six patients who received amikacin therapy.