The environmentally responsive biodegradative arginine (adi) and lysine (cad) decarboxylases are maximally induced when Escherichia coli is cultured under acidic, anaerobic conditions in rich medium. Previously, transposon mutagenesis led to the identification of hns (encoding H-NS, a histone-like DNA binding protein) as being a trans-acting regulatory factor of both systems. The hns mutants show derepressed expression of adi or cad (i.e., their expression is increased). The effects of the local anesthetics phenethyl alcohol (PEA) and procaine (both environmental perturbants) were investigated with lacZ operon fusions to either adi or cad and their respective hns mutants. These results indicate that wild-type fusion strains are insensitive to either PEA or procaine, but that hns mutants show decreased beta-galactosidase synthesis in the presence of one or both of the local anesthetics. This is the first report of the effect of local anesthetics on hns mutants in this or any other environmentally responsive system.