We have previously demonstrated Pseudomonas aeruginosa resistance selection because of suboptimal carbapenem exposures in an in vitro infection model, but the in vivo relevance of the observations is not well established. In this study, we examined the impact of carbapenem exposures on resistance selection using a neutropenic murine pneumonia model. Neutropenic mice were infected with approximately 10(6) CFU of P. aeruginosa intratracheally. Ten animals each were treated with 400 or 50 mg/kg of meropenem intraperitoneally or placebo every 8 h, given 2 h after infection for 2 to 4 days. Quantitative assessment of bacterial burden in lung tissues was performed at baseline, upon death, or at the end of experiment. Meropenem (400 mg/kg) offered a significant survival benefit, but selective amplification of the OprD(-) mutant population in lung tissue was observed in 20% to 30% of the animals. Our data suggested that suboptimal meropenem exposures might facilitate in vivo selection of resistance in a heterogeneous P. aeruginosa population.