1. Infusions of adrenaline in physiological amounts alter human muscle contractions evoked by nerve stimulation.2. Adrenaline shortens the duration of the slow calf muscle twitch, but has no effect on the fast twitch of adductor pollicis.3. Adrenaline decreases unfused tetanic tension and increases the oscillation of tension in 10/sec tetani of calf muscle and adductor pollicis. The usual rise of tension and decrease in oscillation in unfused tetani (;ramp' phenomenon) is abolished.4. Adrenaline has no effect on maximal tetanic tension or maximal rate of rise of tension in a fused tetanus of adductor pollicis.5. The effects of adrenaline on human muscle are due to stimulation of beta-adrenotropic receptors, for they are abolished by the beta-adrenotropic antagonist DL-propranolol (but not by D-propranolol), and are mimicked by isoprenaline but not by noradrenaline.6. The effect of adrenaline on adductor pollicis is abolished by local beta-blockade of one arm with intra-arterial DL-propranolol, indicating that the responsible beta-receptors lie peripherally.7. The changes in muscle contraction observed cannot be explained by altered muscle temperature, for this falls during adrenaline infusion; nor are they due to an action on neuromuscular transmission, for these small doses of adrenaline do not affect the muscle action potential. The evidence points to a direct action of adrenaline on muscle.