The effects of a bolus injection of diisopropyl phenol 2 mg kg-1 i.v. and an infusion (150 micrograms kg-1 min-1 for 30 min and 75 micrograms kg-1 min-1 thereafter) on the pharmacodynamics and dose-response curves of atracurium and vecuronium were studied in 52 healthy (ASA I or II) patients. Under anaesthesia with diisopropyl phenol, the cumulative dose-response curves of both myoneural blocking drugs were shifted to the left when compared with previously reported results. However, there was no clinically significant change in the pharmacodynamics of either drug after the bolus injection of the blocking drug. Diisopropyl phenol 2 mg kg-1 i.v. administered during steady-state infusions of vecuronium and atracurium increased the depth of the constant neuromuscular blockade. When diisopropyl phenol alone was given to four patients there was no change in the twitch height. These results show that diisopropyl phenol, given according to the regimen in this study, potentiates both of the neuromuscular blockers studied but does not prolong the duration of action of either drug. During the infusion of diisopropyl phenol, vecuronium was found to be five times more potent, to have a more rapid onset time, and to be shorter acting than an equipotent dose of atracurium.