1. Muscle mass action potentials (M waves) were evoked by supramaximal single shocks to the ulnar nerve given at 5-10 s intervals throughout sustained isometric maximal voluntary contractions (m.v.c.) of the adductor pollicis and first dorsal interosseous muscles. Both muscles were fatigued simultaneously. Recordings were made from the muscle surface and also intramuscularly. 2. During a maximal contraction lasting for 60 s there was 30-50% loss of force. No decline was observed in intramuscularly recorded M wave amplitude, while the areas of the total and half M wave forms increased due to a slowing in conduction velocity. The area measured over a fixed time period declined. No evidence was obtained that these M wave potentials were contaminated by electrical activity arising in adjacent muscles. The size of the single unit potentials appeared to remain unaltered during maximal voluntary activity. 3. We conclude that neuromuscular block is not a cause of force loss during this type of fatiguing voluntary contraction.