To characterize the pharmacokinetics of gentamicin during and after hemodialysis (using polysulfone Fresenius F-80 membranes (Fresenius USA, Inc, Walnut Creek, CA), surface area 1.6 m(2)), eight patients with end-stage renal disease undergoing chronic hemodialysis receiving the drug for therapeutic indications were enrolled. Intradialytic gentamicin half-life, clearance, and amount of gentamicin recovered during a hemodialysis session were also determined. Serum gentamicin concentrations were analyzed using fluorescence polarization immunoassay. The amount of gentamicin recovered was 64.3 +/- 14.4 mg, whereas the intradialytic gentamicin half-life was 2.24 +/- 0.83 hours, with a clearance of 116 +/- 9 mL/min. Gentamicin concentrations rebounded by 27.86% +/- 16.4% at 1. 5 +/- 0.52 hours after the end of the hemodialysis session. The decrease in gentamicin concentrations comparing maximum rebound to prehemodialysis concentrations was 53.54% +/- 9.97%. A variable yet substantial amount of gentamicin is removed during hemodialysis using F-80 membranes; hence, supplemental doses are necessary to avoid potential treatment failures. The supplemental doses of gentamicin calculated based on gentamicin concentrations obtained immediately postdialysis could be overestimated if the postdialysis rebound concentrations are not considered. A dosing regimen is suggested using the pharmacokinetic parameters defined by the present study and population estimate of volume of distribution.