A novel method of characterization of viscoelastic longitudinal stresses in electrospun jets of semi-dilute and concentrated polymer solutions and melts is introduced. The measured longitudinal stresses at the beginning of the thin jet region in the jets of a 6 wt% aqueous solution of polyethylene oxide (M-w = 400 kDa) were of the order of 100 kPa, which is two orders of magnitude larger than in any other free viscoelastic jets issued from nozzles and orifices. This is attributed to elongation-driven stretching of polymeric liquids in the transition zone, between the preceding modified Taylor cone zone and the beginning of the thin jet region, where the stretching rates are of the order of 100-1000 s(-1). The Rouse relaxation times found were in the range of 3-8 ms, and the moduli of elasticity were of the order of 100 Pa. A novel explanation of the reasons for the formation of the straight sections in the electrospun jets is proposed. The straight sections are stabilized by the high initial longitudinal stresses in the jet generated due to strong electrically driven stretching in the transition zone. The further electrically driven stretching in the jet (after the transition zone) is relatively weak, and viscoelastic Rouse relaxation prevails. The relaxation distance of the longitudinal stresses along the jet increases with the applied voltage (which generates higher initial stresses in the transition zone) and thus the length of straight section of the jet should increase as the applied voltage increases. The results also point at an opportunity to develop an elongational rheometer for concentrated polymeric systems with stretching rates of the order of 100-1000 s(-1). The proposed rheometer employs excitation of electrically driven jets by single lateral pulses, and observation of the pulse propagation and widening along the jet. This reveals the level of the longitudinal stresses along the jet and allows evaluation of the viscoelastic Rouse relaxation time, modulus of elasticity and the elongational viscosity in the jet. (c) 2008 Elsevier Ltd. All rights reserved.