In 48 patients anaesthetized with nitrous oxide-oxygen and hexobarbitone the neuromuscular (n-m) block (greater than or equal to 95% depression) produced by roughly equipotent doses of d-tubocurarine (dTC), gallamine (GALL), pancuronium (PANC) or alcuronium (ALC), respectively, was antagonized by 10 mg of pyridostigmine (P) applied intravenously 35-460 min after the relaxant at variable levels of spontaneous recovery from n-m block. Muscular reactions to tetanic stimulation (30 to 400 Hz, 4-5 s each) of the ulnar nerve transmitted by a force-displacement system served as a measure for calculating the relative amount of n-m receptors liberated from relaxant molecules. Within 3-10 min after its injection P increased the number of relaxant-free n-m receptors by 16 +/- 6% (M +/- SD). Thereafter recovery progressed at similar speed as before. Reinjections of 5-10 mg P were comparably as effective as the first injection. No correlations were to be found between the effectiveness of P and the dose of relaxant applied (r = 0,12 to 0,27), the level of recovery reached before P (r = 0,32), or the time at which P was injected after the relaxant (r = -0,39), respectively. However, the amount of receptors liberated by P decreased with increasing recovery from n-m block and with increasing time interval between the relaxant and the antidote injection. P was significantly more effective (P less than 0,01), when applied within 150 min after the relaxant than at applications after that time. The relative number of receptors liberated by this drug was insignificantly larger in the PANC-and GALL-block than in the ALC-and dTC-block.