The pharmacodynamics and pharmacokinetics of the two most recent aminosteroid neuromuscular blocking drugs to become available, rapacuronium bromide (Org 9487) and rocuronium bromide are reviewed. Two new classes of drug with neuromuscular blocking properties, the bis-tetrahydroisoquinolinium chlorofumarates and the tropinyl diester derivatives are introduced. Comparisons between these drugs and mivacurium and cisatracurium are made. Rapacuronium 1.5 mg kg(-1) (ED95 1 mg kg(-1)), produces maximal neuromuscular block in 54 s. Time to recovery of the train-of-four ratio to 0.7 is achieved within 20 min after neostigmiine 0.05 mg kg(-1) given at 2 min. The plasma clearance of rapacuronium is 7-8 ml kg(-1) min(-1). Rapacuronium undergoes hepatic metabolism: no prolongation of effect has been reported after a single bolus or a short infusion in patients with hepatic or renal failure. Org 9488 is the 3-desacetyl metabolite of rapacuronium, which has neuromuscular blocking properties. Its much lower clearance (1.28 ml kg(-1) min(-1)) and plasma equilibration constant (0.105 min(-1)) may limit the prolonged use of rapacuronium. Rocuronium given at 2xED95 produces maximal neuromuscular block in 1 min. Spontaneous recovery of the train-of-four ratio to 0.7 takes over 40 min. Rocuronium has a plasma clearance of 4 ml kg(-1) min(-1). Its pharmacodynamics are altered in hepatic and renal disease. A number of anaphylactoid reactions to rocuronium have been reported recently. The bis-tetrahydroisoquinolinium chlorofumarate GW280430A has an ED95 of 0.19 mg kg(-1). Given at three times this dose, onset of neuromuscular block occurs within 100 s; the duration of block is 8-9 min. Following a 2 h infusion, the recovery index does not seem to be increased. Early studies suggest that this drug has no adverse cardiovascular or respiratory side-effects. The tropinyl diester derivative G-1-64 will produce 80-90% neuromuscular block in less than 2 min using 3xED80. Ninety per cent recovery of the first twitch of the train-of-four occurs after 5-7 min using one ED80. A recovery index of less than 2 min has been reported in rats. All the tropinyl diesters appear to produce vagal block.