An in-vitro model which simulates in-vivo pharmacokinetics was used to compare the efficacy against Pseudomonas aeruginosa of dosing regimens of gentamicin which achieve different peak/trough concentrations but use the same total dose over 24 h. First exposure to gentamicin produced a rapid bactericidal effect which was proportional to the initial peak concentration. Subsequent doses of gentamicin produced a smaller bactericidal effect. Regrowth occurred with all dosing regimens, even after very high initial concentrations (26 mg/L). The time to reach bacterial counts above starting values was prolonged in relation to peak concentrations. Regrowth was also demonstrated in continuous infusion experiments which maintained very high concentrations (26 mg/L), although an inhibitory effect was evident compared with single dose experiments and the experiments mimicking in-vitro pharmacokinetics. There was little evidence of a post-antibiotic effect. The data supports the use of larger initial and longer interval bolus dosing compared with current recommendations.