1. The effect of intravenous aminophylline on the contractility of adductor pollicis has been studied in three subjects both in the fresh state and following the induction of muscle fatigue.
2. Aminophylline had no influence on the frequency—force relationship and relaxation rate of adductor pollicis in the fresh state.
3. Fatigue resulted in a selective depression of the force response to low and moderate frequencies of stimulation and a slight effect on maximum force production 10 and 35 min afterwards.
4. Aminophylline given prior to, during and/or after fatigue did not influence this selective low-frequency fatigue at 10 or 35 min.
5. Aminophylline at the concentrations obtained has no significant effect on muscle contractility or fatiguability in man.