We previously showed that the kaolin-induced writhe reaction exhibits 24 h variation with a peak at the end of the resting period (14:00-18:00 h) in mice maintained under light from 07:00 to 19:00 h. In this study, we used this model to evaluate the administration-time-dependent (chronopharmacodynamic) effect of indomethacin. Indomethacin (0.5 mg/kg) was given orally to mice at 02:00, 08:00, 14:00, or 20:00 h, and the suppressive effect on kaolin-induced writhing was determined after each timed dosing. After dosing at 08:00 h, indomethacin remarkably reduced the number of writhes during the critical span of 14:00-18:00 h--the time when writhing reaction was greatest during the 24 h, while the suppressive effect of the medicine after dosing at the other clock times was relatively small. These data suggest the analgesic effect of indomethacin in mice with the kaolin-induced writhing is greater after dosing in the early resting period, which is similar to that reported in patients with nocturnalpain. The kaolin-induced pain mouse model seems to be useful for the chronopharmacodynamic evaluation of analgesic agents.