There have been several reports that insomnia occurs in some patients who receive ofloxacin. Since almost no experimental data on ofloxacin-induced insomnia were available, this study was conducted for the evaluation of ofloxacin effects on sleep parameters in mice. In Experiment 1, mice were pretreated with ofloxacin (20 or 40 mg/kg IP) or saline 15 minutes before sodium pentobarbital (35 mg/kg IP). Experiment 2 was carried out in two days. On the first day mice were treated twice, in the morning and in the evening, with ofloxacin (20 or 80 mg/kg IP) or saline. On the second morning, mice were pretreated with the same doses of ofloxacin or saline 15 minutes before sodium pentobarbital (35 mg/kg IP). Sleep latency and sleeping time were recorded in each experiment. Results showed that ofloxacin had no apparent effect on sleep latency, but caused a shortening in sleeping time. However, this effect was significant only in the 40 and 80 mg/kg ofloxacin-treated groups.