The concentrations of cefoxitin in serum and cerebrospinal fluid (CSF) were measured simultaneously in three groups of patients, 12 with aseptic meningitis (group 1) and 17 and 14 with bacterial meningitis (groups 2 and 3). The patients in group 1 received a single intravenous dose of 2 g of cefoxitin without other antimicrobial therapy. In addition to conventional doses of ampicillin or benzyl penicillin, patients in groups 2 and 3 received repeated infusions of 2 g of cefoxitin every 4 h for the first 3 or 4 days of the study and again on day 10. Additionally, group 3 received probenecid in a loading dose of 1 g followed by 0.5 g every 6 or 8 h. Concentrations of cefoxitin in CSF and serum were determined 1 or 2 h after infusion in group 2 and 2 h after infusion in group 3. The concentrations of cefoxitin in CSF did not reach detectable levels (1.56 microgram/ml) in 11 of the 12 patients in group 1. A level of 2.8 microgram of cefoxitin per ml of CSF was found, with an accompanying level of 30 microgram/ml of serum, in patient 12. In the group 2 patients with bacterial meningitis, the mean CSF concentrations were 3.3, 4.7, and 2.9 microgram/ ml on days 1, 3, and 10 of treatment, with simultaneous serum levels of 8, 9, and 8 microgram/ml. At similar times periods, the mean levels of cefoxitin in group 3 patients (with concomitant probenecid) were 8.6, 12.3, and 4.3 microgram/ml of CSF and 57, 35, 27 microgram/ml of serum.