The purpose of this study was to investigate the influence of hypothermia on the recovery time after either vecuronium or atracurium given by repeated bolus injection, and to determine the adequate time of administration of neuromuscular blockers during cardiopulmonary bypass. Two groups of 10 patients each were studied before(Tl), during(T2) and after(T3) hypothermia. Following an initial intravenous bolus dose of 2X ED95 of either vecuronium(Group I) or atracurium(Group II), repeated bolus dose of ED95 was injected whenever twitch tension had returned to 20% block of train-of-four response before bypass of control As a result, the action duration of vecuronium(from injection of loading dose to twitch tension=20%) was 29.5±5.5 mins, 103.1±12.0 mins and 70.8±9.9 mins before, during and after hypothermia, respectively. The eorresponding values for atracurium were 26.3±5.1 mins, 85.7±11.4 mins and 42.5±5.1 mins. We conclude that hypothermia increases significantly the duration of action of both muscle relaxants(P<0.001) and vecuronium is influenced by hypothermia more than atracurium. The rate of recovery of twitch height after hypothermia is more rapid in atracurium than vecuronium(P<0.001). Single bolus injection of ED95 of vecuronium administered just before cardiopulmonary bypass is suitable for maintaining the neuromuscular block throughout cardiopulmonary bypass.