Tensile tests performed at different strain rates on HDPE geomembranes show that both the strength and the stiffness of the geomembrane decrease with a decrease in the strain rate. The quantification of the strain-rate dependent stress-strain response of HDPE geomembranes is crucial for the design of any geomembrane structure of which the geomembranes are strained at a lower rate than the rate at which laboratory tests are performed. Two simple mathematical models for the strain-rate dependent stress-strain behaviour of HDPE geomembranes under uniaxial loading conditions are presented. The models also provide a method for estimating the initial tangent modulus of the geomembrane. All the model parameters for both the models can be derived from uniaxial tensile tests performed at different strain rates. (C) 2004 Elsevier Ltd. All rights reserved.