A novel elongational device is used to investigate the capillary thinning process of threads of dilute and semidilute aqueous polymer solutions. It is shown that the end regions of the threads do not play an essential role in the thinning process, so that a simple theory describing the self-thinning of the liquid thread is appropriate to describe the experiments with polymer solutions carried out with this device. Aqueous solutions of four different polymers (all polyacrylamide based) are studied using the elongational device. It is shown that the elasticity of the polymers is dominated by polyacrylamide chains and that the effect of branching and topological structure of macromolecules is negligible. The time-dependent decrease of the thread diameter can be divided into two stages. The first is a viscoelastic one where macromolecular coils are stretched by die elongational how, and the second is a quasi-Newtonian one, where full stretching has already been achieved, resulting in a very high but constant elongational viscosity. At the first stage the rheological behavior of the solutions studied is characterized by a constant relaxation time, whereas at the second one it is by a constant elongational viscosity. For polymer macromolecules of relatively low stability, mechanical degradation of the molecules is found during their stretching in a self-thinning capillary thread. (C) 2000 The Society of Rheology. [S0148-6055(00)01003-8].