The thermogenic beta 3-adrenoceptor agonist BRL 26830A has been shown to increase weight loss in dieting subjects but tremor was a frequent adverse effect. We have investigated the magnitude and nature of this tremor after a single oral dose in 18 subjects. Two complementary techniques were used to attach the recording apparatus to the subjects to give both isotonic and isometric measures of tremor. Increases of 84% and 40% respectively were found due to exaggeration of physiological tremor presumably mediated through concomitant beta 2-adrenoceptor stimulation. The use of beta 3-adrenoceptor agonist drugs in the treatment of obesity may increase but the development of an agent without tremor inducing properties would be an obvious advantage.