The pharmacokinetics of alfentanil were studied after two different methods of cardiopulmonary bypass (CPB) which used either a normothermic technique or a hypothermic technique with body temperatures below 30 degrees C. A group of surgical patients who did not undergo CPB were also studied. All patients (n = 36) received alfentanil 50 micrograms/kg as a rapid infusion over 10 min and plasma levels were measured for the following 8 h. The volume of distribution in steady state (Vdss) and the volume of central compartment (Vc) were significantly greater in the CPB groups (P = 0.009 and P = 0.04), compared to the nonbypass group. However, the elimination half-life of alfentanil (t1/2 beta) (mean, 95% confidence interval) did not differ significantly between the normothermic (134 min, range 104-172), hypothermic (143 min, range 111-184), and nonbypass (111 min, range 86-142) groups. Between the normothermic and hypothermic CPB groups no significant differences in Vdss, rate of clearance (Cl), or t1/2 beta were found. The pharmacokinetics of alfentanil showed fewer changes after CPB in this study than have been reported previously. Furthermore, different temperature management protocols during CPB did not influence alfentanil elimination.