1. The effect of intravenous aminophylline on theophylline-induced tremor were studied in six hyperthyroid patients before and after treatment to euthyroidism. 2. Baseline tremor power was significantly greater in the hyperthyroid compared with the euthyroid state (P less than 0.05). 3. Aminophylline produced a significantly greater increase in tremor power in the hyperthyroid compared with euthyroid state. 4. There was a significant increase in plasma theophylline clearance and reduction in t1/2 elimination when patients were hyperthyroid but volume of distribution did not change. 5. Intravenous aminophylline potentiates the tremor produced in hyperthyroidism despite an increase in plasma theophylline clearance. A pharmacodynamic mechanism is therefore likely.