Attempts have been made to correlate the duration of the local anesthetic effect with the amount of the local anesthetic compound remaining at the injection site at different times after an injection. In addition to delaying the absorption of the local anesthetic, adrenaline but not a vasopressin-derivate decreased the minimal anesthetic concentration of the local anesthetic agent. It is assumed that the pH value of the tissues at the injection site was kept down through the metabolic effects of adrenaline, thereby transforming the local anesthetic agent into cations, which is the form in which the amide-type of local anesthetics mainly exerts their nerve blocking activity.