An experimental protocol to evaluate the structured biomass model proposed by Lavallee (Lavallee, Lessard, and Vanrolleghem, J Environ Eng Sci 2005;4:517532) is presented. The protocol was devised to induce transient behavior and characterize the evolution of several internal biomass components. The proposed model is based on biochemical principles, and was fitted to the collected data. In these experiments, it was observed that filling the storage capacity of cells leads to special transient behavior, including a temporarily reduced metabolic activity. The model-based interpretation of the results showed that the observed transient behavior can be explained by cross-regulation of carbon and nitrogen metabolism. Hence, according to an extensive literature review, the cross-regulation of carbon and nitrogen can be used to model some observed transient behaviors and regulation of the storage process in activated sludge. (C) 2011 American Institute of Chemical Engineers Biotechnol. Prog., 2011