In 9 patients, who underwent lumbar drainage after transsphenoidal surgery for pituitary adenoma, we collected 69 samples of CSF and plasma, respectively. The concentration of ofloxacin was measured bacteriologically. Considering the ofloxacin CSF and plasma levels related to time some aspects of pharmacokinetics are discussed. With maximum CSF concentrations coming up to 40% of plasma levels liquor penetrability is quite good. Ofloxacin seems to be able to prevent (postoperative) meningitis, even with grampositive Cocci.