Pharmacokinetics of ceftriaxone after a single dose of 50 or 75 mg/kg were determined in 30 pediatric patients with bacterial meningitis. Data for doses of 50 and 75 mg/kg, respectively, were as follows (mean ± standard deviation): maximum plasma concentrations, 230 ± 64 and 295 ± 76 ,ug/ml; elimination rate constant, 0.14 ± 0.06 and 0.14 ± 0.04 h-1; harmonic elimination half-life, 5 infants, aged 6 weeks to 2 years, who were receiving conventional therapy for documented bacterial meningitis at Arkansas Children's Hospital, Little Rock. Written informed consent was obtained from the parents of all participants. Between days 2 and 5 of therapy, when infection was judged to be under adequate control, and without alteration of the antimicrobial regimen, a single dose of ceftriaxone was administered intravenously over a 10-min period. The study was randomized so that half of the patients received 50 mg/kg and half received 75 mg/kg for this one infusion.Plasma samples were obtained just before infusion and at 15, 30, and 60 min and 2, 4, 6, and 10 h after infusion. A lumbar puncture was performed 1 to 6 h after drug administration, and a sample of CSF was obtained for analysis of ceftriaxone concentration.Susceptibility studies. Mean inhibitory concentrations (MICs) of ceftriaxone for each pathogen were determined by standard microtiter broth dilution (6). An inoculum of 105 organisms per ml in logarithmic growth phase was introduced into wells containing appropriate nutrients for that organism and serial dilutions of ceftriaxone. Cftriaxone concentrations. Plasma and CSF concentrations of ceftriaxone were analyzed by highpressure liquid chromatographic techniques (9). To monitor ceftriaxone levels on a daily basis for patients receiving this investigational antibiotic, microbiological methodology was employed; briefly, this was a standard agar well diffusion assay in which susceptible Escherichia coli is used after dilution of the specimen with pooled plasma (1).Pharmacokinetic determinations. The elimination rate constant, 1, was determined from the regression Clinical and laboratory studies. Patients were carefully monitored for adverse reactions during infusion by one of the investigators and followed during the duration of hospitalization. In addition, laboratory parameters for bone marrow, renal, or hepatic toxicity were obtained preinfusion and at 2 and 4 days. These included CBC, blood urea nitrogen, creatinine, urinal-