The kinetics of the reaction between bilirubin and bovine serum albumin have been re-investigated at 20 degrees C. The results of previous authors concerning both human and bovine serum albumin were largely confirmed, namely the existence of two first-order configurational changes after a primary complex is formed in a fast, bimolecular step. From the kinetic behaviour it is concluded that this primary complex is present in non-negligible concentration after equilibrium is reached, and that it exchanges bilirubin with the surroundings with a rate constant of at least 23 s-1. This also means that the secondary complex, and, possible, also the final product, are in dynamic equilibrium with each other and with the primary complex, and, therefore, only the sum of their formation and dissociation rate constants can be measured. The dependence of the two observable relaxation times on pH does not parallel the N leads to B transition. On the other hand, a pH jump between 7.4 and 9.0 is assumed to monitor the N leads to B transition, both the free albumin and the complex in its various forms undergoing this transition at identical rates. This transition, although influencing the absorptivity of the complex, was found not to influence the strength of the binding site.