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High-level chloramphenicol (CM) resistant variants of Pseudomonas aeruginosa were isolated after culture of the wild-type (WT) strain in broth containing high concentration of the drug. These variants exhibit reduced ability to accumulate several amino acids. The extent of reduction in transport capacity is a function of the concentration of CM in which the variants are grown. Respiratory activity is not reduced in these strains. Amino acid uptake is not affected by the presence of CM during assay. An isogenic strain carrying a plasmid coding for CM resistance does not show this response to CM. Transport capacity is restored to the WT level in CM-sensitive revertants. These results suggest that the acquisition of CM resistance in P. aeruginosa is associated with a fundamental alteration in membrane permeability which is regulated by metabolism in the presence of the drug. The ramifications of this for the study of CM action and resistance are discussed.