The relationship between plasma propranolol concentration and relief of essential tremor was examined in 11 patients during treatment with oral racemic propranolol in doses of 30 to 640 mg/day. Although propranolol decreased tremor in all 11 patients, the degree of improvement varied widely in individuals (mean 51%, range 25--90%), and was not related directly occurred at plasma propranolol concentrations below 20 ng/ml (0.077 mumol/l) and, in three others, below 40 ng/ml (0.154 mumol/l). It is concluded that the optimum response of essential tremor to propranolol is achieved at relatively low plasma propranolol levels, levels which are obtained by daily propranolol doses of 120--240 mg.