Abstract: Objective: To study the influence of acute normovolemic hemodilution on the pharmacokinetics of fentanyl in intracardiac operation.Methods: Sixty patients scheduled for mitral valve replacement were randomly divided into acute hemodilution group(n=30) and control group(n=30).The blood samples from the radial artery following injection of fentanyl 60 μg/kg were collected to determine the plasma fentanyl concent ration by gas chromatography-mass spectrum method.The pharmacokinetic parameters were calculated with 3P87 computer program.Results: Compared with the control,the plasma fentanyl concentration in the acute hemodilution group decreased markedly;t_1/2α(44.13±18.80)min vs(7.57±5.9)minand t_1/2β(440.55±231.32)min vs(1 213.73±148.85)min prolonged significantly(P<0.001);Vc(14.135±4.850)L vs(9.477±2.321)Land Vd(10.450±3.153)L vs(4.187±1.045)L increased by 1.5 and 2.5 times,respectively(P<0.001).In contrast,CL was similar between the two groups(P>0.05).Conclusions: Acute normovolemic hemodilution in open heart surgery can influence the pharmcokinetics of fentanyl by increasing the volume of distribution and prolonging the elimination.