Prolonged distribution time has been noted for high-dose (7 mg/kg) gentamicin. Higher doses are used for extended-interval aminoglycoside therapy (EIA). The authors investigated whether the increase in distribution time was proportional to the dose of gentamicin. Twelve healthy volunteers were given low (LD, 2 mg/kg), medium (MD, 4.5 mg/kg), and high (HD, 7 mg/kg) doses of gentamicin in a randomized, crossover fashion. Gentamicin was infused over 30 minutes, with 15 concentrations obtained over 8 hours after each dose. Data were fit to a two-compartment pharmacokinetic model. Distribution half-life for HD (31.1 +/- 5.7 min) differed significantly (p < 0.05) from LD (22.4 +/- 6.1 min) and MD (23.8 +/- 5.1 min) with no significant difference being seen between LD and MD. This study verifies that when using EIA dosing with HD gentamicin, sampling within 90 minutes after the beginning of the infusion provides information that leads to overestimation of peak serum concentration/minimum inhibitory concentration and inaccurate calculation of pharmacokinetic parameters.