Dynamic and steady state flow measurements and shrinkage experiments have been carried out to investigate the influence of blend composition on viscous and elastic properties of a new type of polyolefin : high density polyethylenes of high and low molecular weight obtained via metallocene catalysts. The polyethylenes are characterized by narrow molecular weight distribution and absence of branching. A model based on the additivity of molecular weight and free volume has been used to fit the complex and steady state viscosities vs composition data, at constant frequency or shear rate, which show a large positive deviation from linearity. The single correlation between G’ and G " and the small effect of blend composition on dimensional stability correspond to the behaviour expected for a miscible blend. ’Sharkskin’ and ’slip-stick’ effects observed in extrusion process of the high molecular weight sample disappear for blends of 50% and higher content of low molecular weight polyethylene.