Abstract: AIM To investigate the influence of dosing time on acute toxicity, action and pharmacokinetics of tramadol in mice to find the circadian rhythms of the drug. METHODS Tramadol was given (ip) to mice at 08:00, 12:00, 16:00, 20:00, 24:00, 04:00 and the variations of their acute mortality, responses to hot plate and tail-pressing stimulation and pharmacokinetics were investigated. RESULTS Significant circadian rhythm was found for the acute toxicity of tramadol in mice and the highest mortality was found when the drug was administered in the middle dark phase. The rhythm of drug toxicity coincided nicely with the rhythm of plasma drug level. Tramadol effect was demonstrated to vary circadianly and the highest effect occurred in the late dark phase. Dosing time dependent kinetics of tramadol was also found in mice. CONCLUSION Significant circadian rhythm of the acute toxicity, action and pharmacokinetics was found in mice when tramadol was given at different times of the day.