An experimental investigation has been carried out to study the rheological behaviour of ethylene propylene diene rubber (EPDM) compounds in extrusion containing blowing agent. The cell morphology development and rheological properties have been studied for gum and carbon black filled systems with variation of the blowing agent, extrusion temperature and shear rate. The apparent shear stress (tau(wa)), apparent viscosity (eta(a)) die swell (%) and total extrusion pressure (Delta P) of the carbon black filled EPDM rubber compounds have been determined using the Monsanto processability tester (MPT). The effect of the curing agent and blowing agent (DNPT) on the rheological properties of the compound has also been studied. There is a significant reduction in stress and viscosity with the blowing agent in the presence of curing agent compared with the absence of curing agent. The viscosity reduction factor is found to be dependent on the blowing agent loading, shear rate and temperature. (C) 1997 Elsevier Science Ltd.