The effects of suxamethonium and tubocurarine on the refractoriness of neuromuscular transmission were studied in 21 anaesthetized adult subjects under various levels of neuromuscular block. The ulnar nerve was stimulated every 12 s with twin supramaximal stimuli 4 ms apart. At any level of block, the refractory fraction (the fractional decrement of the compound electromyographic response of the adductor pollicis to the second twin stimulus, relative to the response to the first) was used to quantify neuromuscular refractoriness. The magnitude of block was determined by the response to the first stimulus. Correlation between refractoriness and the degree of block was sought. Without block, neuromuscular transmission averaged 23% (SEM 4) refractory with this twin interval. The refractory fraction was increased markedly by suxamethonium, reaching 0.69 (SEM 0.1) at 50% block. Complete refractoriness occurred during 25--75% block in eight of 11 instances. Tubocurarine did not significantly alter refractoriness, paired responses to the twin stimuli decreasing proportionately during block.