The three-dimensional neck propagation behavior of a glassy polymer under tension, which obeys a constitutive equation based on the molecular chain network theory, was investigated numerically. An experimental observation of tension in polycarbonate (PC) specimens was performed. A hybrid identification procedure of the parameters in the constitutive equation was developed by employing the least mean squares method with an evaluation function which is defined by the square of the difference of the computational and experimental results for nominal stress at specific deformation stages. Application of the proposed procedure to PC led to a very smooth convergence process and accurate results. The validity of the results was also examined through a comparison of the calculated lateral contractions of the specimens at different points in different stages of the deformation with those obtained by experiments. (C) 1997 Elsevier Science Ltd.