The penetration of ciprofloxacin into the cerebrospinal fluid (CSF) in 25 patients with non-inflamed meninges and in 9 patients with inflamed meninges was studied. In the patients with non-inflamed meninges plasma and CSF were obtained 1-10 h after the second dose of ciprofloxacin and in the patients with inflamed meninges 1, 2, 3, 5, 7 and 9 h after the second dose. In the first group (non-inflamed meninges) data from 6 patients were obtained 4, 5 and 6 h post-dose. Mean ciprofloxacin concentrations in the CSF ranged from 0.073 mg/l to 0.106 mg/l during this observation time, while in the second group (inflamed meninges) they ranged from 0.089 to 0.260 mg/l. These results demonstrate that ciprofloxacin diffuses into the CSF at concentrations which exceed the MICs of Neisseria meningitidis and most gram-negative aerobic bacilli.