Uncontrolled clinical studies have suggested that primidone may be effective in reducing essential tremor thus providing a valuable alternative to beta-adrenoreceptor antagonists which are currently the drugs of first choice. A double blind, placebo controlled trial of primidone in essential tremor of the hands and head was carried out using both clinical and objective methods of assessment. Primidone was significantly superior to placebo in reducing the magnitude of hand tremor, its efficacy being comparable to that of propranolol. In two patients tremor was reduced to non-symptomatic levels, an effect rarely seen with propranolol. No consistent attenuation of head tremor was found. There was no correlation between serum primidone or derived phenobarbitone concentrations and the reduction of hand tremor. An acute toxic reaction to an initial small dose (62.5 mg) of primidone was seen in six of 22 patients.