The quorum-sensing (las and rhl) systems play critical roles in the pathogenicity of Pseudomonas aeruginosa and its synthesis of the important biosurfactants, rhamnolipids. In this work, P. aeruginosa PAO1 and its rhlI and rhlR null mutants were used to study the degradation and synthesis kinetics of the rhl system's autoinducer PAI2 (N-butanoyl-homoserine lactone). The two mutants, lacking the ability of synthesizing PAI2 or RhlR protein, produced insignificant amounts of rhamnolipids while having similar growth profiles as the wild-type culture. The regulatory RhlR:PAI2 complex is thus essential to rhamnolipid synthesis. In batch culture of the wild-type PAO1, the autoinducer PAI2 concentration increased along cell growth, especially during the transition from exponential-growth phase to stationary phase, and began to decrease after entering the stationary phase. The decrease in the stationary phase resulted from a faster PAI2 degradation than its synthesis. The degradation kinetics was studied using PAI2-containing supernatants (from centrifuged broth of wild-type culture) with and without the rhlI(-) mutant cells incapable of PAI2 synthesis. Being insignificant in the cell-free systems, PAI2 degradation was found predominantly cell-associated and could be described empirically by the first-order, exponential decay kinetics with the best-fit degradation constant (k(d)) of 0.195 h(-1). When similarly modeled with a first-order kinetics, PAI2 synthesis in stationary-phase wild-type culture was derived to have a synthesis constant (k(s)) of 0.189 h(-1). The PAI2 concentration in batch cultivation of the rhlR(-) mutant also showed an increase-then-decrease profile. However, the maximum PAI2 concentration was about one third of that from the wild-type culture. The constitutive rate of PAI2 synthesis was therefore significantly lower than the rate attainable with active auto-induction by RhlR-PAI2 complex.