The penetration of imipenem and cilastatin into the cerebrospinal fluid (CSF) was determined in patients with bacterial meningitis. Four 1000 mg doses [corrected] of both imipenem and cilastatin were infused intravenously over 20-30 min at 6 h intervals, first between days 2 and 4, and again, whenever possible, between days 11 to 20, in 12 patients with bacterial meningitis undergoing treatment with other antibiotics. Concentrations of imipenem and cilastatin in serum and cerebrospinal fluid samples obtained either at 60, 90 or 120 min following the fourth dose were measured by high pressure liquid chromatography. Concentrations of imipenem in CSF ranged from 0.5 to 11 mg/1 and concentrations of cilastatin ranged from 1.1 to 10.5 mg/1, depending on the sampling time and the time elapsed since the onset of the disease.