Abstract: A method of determination of extensional viscosity and elasticity of a molten polymer or other fluid comprises making pressure measurements on flows of the fluid through an apparatus which includes a fluid delivery system communicating with an open ended die, the cavity of the die being shaped so as to include a converging portion and a diverging portion. The diverging and converging portions preferably intercommunicate with each other through a die cavity portion of uniform cross section. In a preferred form of the method there is also determined the shear viscosity of the fluid. The converging and diverging portions of the die are preferably symmetrically shaped with respect to each other.