The development of tolerance in Pseudomonas putida DOT-T1 to toluene and related highly toxic compounds involves short- and long-term responses. The short-term response is based on an increase in the rigidity of the cell membrane by rapid transformation of the fatty acid cis-9,10-methylene hexadecanoic acid (C17:cyclopropane) to unsaturated 9-cis-hexadecenoic acid (C16:1,9 cis) and subsequent transformation to the trans isomer. The long-term response involves in addition to the changes in fatty acids, alterations in the level of the phospholipid polar head groups: cardiolipin increases and phosphatidylethanolamine decreases. The two alterations lead to increased cell membrane rigidity and should be regarded as physical mechanisms that prevent solvent penetrance. Biochemical mechanisms that decrease the concentration of toluene in the cell membrane also take place and involve: (i) a solvent exclusion system and (ii) metabolic removal of toluene via oxidation. Mutants unable to carry out cis → trans isomerization of unsaturated lipids, that exhibit altered cell envelopes because of the lack of the OprL protein, or that are unable to exclude toluene from cell membranes are hypersensitive to toluene.