The potency, histamine releasing ability, cardiovascular effects, and pharmacodynamics of vecuronium and atracurium were investigated in 64 healthy patients following administration i.v. Cumulative dose-response curves showed vecuronium to be 4.4 times as potent as atracurium. Using the calculated ED90 of each drug (43 micrograms kg-1 for vecuronium and 188 micrograms kg-1 for atracurium), vecuronium had a significantly more rapid onset time and shorter duration of action than atracurium. When three times the ED90 doses were used (129 micrograms kg-1 for vecuronium and 564 micrograms kg-1 for atracurium), no statistically significant differences in the pharmacodynamics were seen between the two drugs, although vecuronium tended to be shorter-acting. There was no clinical evidence of histamine release during the study. No clinically significant changes in arterial pressure or heart rate were seen after the injection of either drug, although vecuronium caused a statistically significant decrease in heart rate (approximately 5%) at 5 and 10 min after administration. Both drugs would appear to have certain advantages over existing non-depolarizing neuromuscular blocking drugs.