The neuromuscular blocking and circulatory effects of a recently synthesized relatively short acting nondepolarizing muscle relaxant, Duador, have been investigated in anaesthetized patients. Duador appears to be a potent muscle relaxant with intermediate duration of action. It had no cumulative effects even after as many as five maintenance doses. Doses of 0.4 mg/kg provided ideal intubating conditions in 2 min, though maximal blockade only developed in about 6 min. Duador was easily antagonized by edrophonium and no signs of recurarisation have been observed. The initial dose of Duador caused a mean 28% increase in heart rate. The increase in heart rate was inversely proportional to the control rate. Repeat doses did not cause further elevation in frequency. Duador had no effect on mean blood pressure.