A mathematical model to study the dynamic behaviour of a city bus equipped with a hydrostatic regenerative braking system has been developed using AMESim software. The hydrostatic circuit comprises a reversible hydrostatic unit and two hydropneumatic accumulators, connected with the conventional powertrain, according to the power assist scheme. The mathematical model of the complete system involves the development of new specific models and the modification of the standard library models with the AMESet editor. Simulations performed with the mathematical model show reductions in fuel consumption of about 20 % for the regenerative braking system under urban driving conditions.