In order to study phenotypic and genotypic alterations in response to selected conditions of stress, Pseudomonas aeruginosa PU21, harboring the multiresistance plasmid, Rip64 (142.5kb), was adapted by serial subcultivation on selective agar plates, amended with either Hg2+, carbenicillin, or gentamicin. Concentrations of the selective agents, to which P. aeruginosa PU21 Rip64 carried plasmid-encoded resistance genes, were close to or slightly above the respective minimal inhibitory concentration. Three different substrains were obtained : R64-M was adapted to Hg2+, R64-C to carbenicillin, and R64-G to gentamicin. All three substrains showed elevated levels of Hg2+ and carbenicillin resistance. Resistance to other antibiotics was either increased, decreased or unaffected. Increased Hg2+ resistance in all adapted substrains was accompanied by higher activity and less stringent induction of mercury reductase, as determined by Hg-volatilization assays with resting cells. No plasmid alterations compared to the nonadapted strain were detected in R64-C, but in R64-M and R64-G plasmid sizes increased by approx. 23 kb, as detected by pulsed field gel electrophoresis and the occurrence of additional DNA fragments after digestions with several restriction endonucleases. The plasmid localized gene rearrangements were accompanied by additional DNA fragments hybridizing with a merA gene probe.