In an effort to develop robust control strategies for the wet-end of papermaking, an experimental program was carried out to model the dynamic behaviour of the filler retention in paper using a Fourdrinier pilot paper machine and a cationic poly(acrylamide)/bentonite retention aid system. A dynamic simulation model of the pilot paper machine was developed from first-principles. To describe the wet-end chemistry effect, the first-pass retention was included in the model as a new independent variable. The important factors affecting the dynamics of the wet-end are first-pass retention and the parameters related to the white water circulation.