In accordance with clinical experience the duration of the action of a local anaesthetic agent, used in dental surgery, changes depending on the time of day. A statistically significant circadian rhythm in the duration of a local anaesthesia, produced by mepivacain, was found in teeth. The longest duration was ascertained at approximately 15.00 h, the shortest at night and in the early morning. These findings coincide with the results of other authors concerning other organs as well.