The characteristics of the myoneural block caused by a new neuromuscular blocking agent, dioxonium (Dx), were evaluated in surgical patients. The force and the corresponding electromyogram (EMG) of the thumb adduction evoked by various modes of ulnar nerve stimuli were measured. Onset, maintenance and disappearance of blockade after sequential administration of Dx were compared with results obtained with d-tubocurarine (dTc) or suxamethonium (Sx). Initially the Dx block was shown to be depolarizing with a negligible fade in the 2 Hz train of four stimuli with a single twitch suppression of 90%. On a weight basis, Dx was found to be about 15 times as potent as dTc in suppressing twitch to the 90% level. During maintenance, the block gained nondepolarizing characteristics with profound fades in the 2 and 50 Hz trains. The transition was associated with tachyphylaxis and with a more pronounced suppression of EMG amplitude than that measured in the twitch force. After a total dose of about 100 microgram/kg of Dx, the sensitivity to Dx again increased and the discrepancy between twitch tension and EMG disappeared. This pattern of changes was also seen with Sx. Spontaneous recovery occurred slightly faster than after dTc blocks of corresponding duration. With neostigmine, reversal was hastened and a full recovery with restitution of prerelaxant twitch and disappearance of fades was reached in about 20 min. Some discrepancy in EMG amplitude and twitch force persisted, however.