lasI- lasRand the rhlI- rhlRquorum-sensing systems in Pseudomonas aeruginosaregulate the expression of numerous cellular and secreted virulence factor genes and play important roles in the development of biofilms. The lasand rhlsystems themselves are known to be directly or indirectly regulated by a number of transcriptional regulators, and consequently, their expression is sensitive to environmental conditions. In this report, the activities of these two quorum-sensing systems have been examined systematically under 46 growth conditions, and the regulation by environmental conditions has been investigated. The relative timing and strength of expression of these two systems varied significantly under different conditions, which contrasts with the notion of a preset hierarchy with these two systems in P. aeruginosa. Depending on the growth conditions, the correlation between each synthase and its cognate transcriptional regulator also varied, suggesting that the transcription of these genes independently allows for further fine tuning of each system. Finally, we observe that the activities of both the lasI- lasRand the rhlI- rhlRquorum-sensing systems were dramatically enhanced in the presence of extracts of sputum samples from cystic fibrosis patients.