A dose finding study was performed for the use of oral tocainide in the treatment of tinnitus. In a single-blind study, increasing doses of tocainide hydrochloride ranging from 0 to 1800 mg and back to 0 mg were administered to 19 patients. Each dose was given for 4 days and the effects as well as the serum concentrations were recorded on the 4th day. In 8 patients the symptoms regressed to a non-troublesome level. In most instances this effect was seen at doses of 900 and 1800 mg, respectively. Side effects were frequent. The most important side effect, which was an absolute indication for termination of the study, was an exanthema that developed in 7 of the patients (37%). Taking into account all side effects observed, it was concluded that no justification exists for doses over 900 mg daily for the treatment of tinnitus. However, in comparison with lidocaine, which can only be given intravenously, the therapeutic score of tocainide is low and in fact tocainide is no satisfactory alternative to lidocaine.