The epileptogenicity of antibiotic drugs represents a clinical problem, and it is well known that the use of penicillin and certain other preparations can induce seizures. In the present study, the authors investigated the epileptogenic properties of different concentrations of 12 commonly used antibiotic medications belonging to seven separate groups. The drugs were tested in the hippocampus, which has a low threshold for the development of epileptiform activity. The hippocampal slice technique, using rat tissue, was employed since absence of the blood-brain barrier allows administration of the drugs in known concentrations. The preparation was exposed to antibiotics in known concentrations and the amplitude and number of population spikes were recorded. Penicillin G was used as a reference substance. Cloxacillin (> or = 1 gm/liter), cephalothin (> or = 1 gm/liter), gentamicin (> or = 80 mg/liter), chloramphenicol (> or = 1 gm/liter), ciprofloxacin (> or = 50 mg/liter), erythromycin (> or = 1 gm/liter), and ampicillin (> or = 1 gm/liter) showed moderate to marked epileptogenic effects, whereas cefuroxime, clindamycin, cefotaxime, vancomycin, and tobramycin had no epileptogenic effects.