There have been several reports that convulsions, although rare, occur in patients who received fluoroquinolones. In this study, conducted for the evaluation of the convulsant action of fluoroquinolones, the effect of ofloxacin and ciprofloxacin on pentylenetetrazol-induced convulsions were investigated in mice. Mice were pretreated intraperitoneally (IP) with saline, ofloxacin (20 or 80 mg/kg) or ciprofloxacin (20 or 80 mg/kg) 30 minutes before subcutaneous (SC) administration of pentylenetetrazol (40 or 60 mg/kg). In another experiment, diazepam (5 mg/kg) was injected (IP) in mice alone or in combination with ofloxacin (80 mg/kg) 30 minutes before pentylenetetrazol (40 mg/kg) administration (SC). In each experiment mice were observed over the following hour for the incidence and onset of clonic convulsions. Results showed that both doses of ofloxacin increased the incidence of clonic convulsions induced by 40 mg/kg pentylenetetrazol. This effect, however was only significant in the higher dose and inhibited by diazepam. On the other hand, a similar proconvulsant effect by ciprofloxacin could not be demonstrated.