Poly(methyl methacrylate) (PMMA) is used to model the relaxation and plastic flow mechanisms of deformation, and the characteristic stress-strain response, relating it to its structure as much as it is known. The scope of this work is to identify and quantify the micromechanisms and the corresponding stress-strain relationships, and to assemble these into a coherent and self-consistent model for the observed mechanical behaviour. Detailed relationship between relaxation strength and time constants has been derived for some of the secondary motions. It is proposed that plastic events occur when the tension in chain segments pulls the chains out of/through constriction points. The scheme for simulating the isothermal true stress-strain curve is carried out under the following limitations: (i) geometrical effects. such as elastic instability and necking, (ii) thermodynamic adiabatic effects, and (iii) structural and kinetic effects, such as may arise from quenching or annealing, are neglected. Qualitative agreement achieved here is considered satisfactory in view of the simplicity of the model and only a few adjustable parameters. (C) 2003 Elsevier Ltd. All rights reserved.