The difficulties of testing brittle heterogeneous materials are considered in the context of obtaining representative data to complement theoretical studies. It is suggested that many of these difficulties can be avoided by using a model material which can be specified numerically and examined by computation. A model of this sort is described, consisting of a plane pin-jointed random network structure of linear elastic brittle members having a range of different strengths and stiffnesses. Results of calculations simulating uniaxial extension tests are given. These show that the model material softens under increasing strain in much the same way as a range of physical materials including rock, concrete, and nonwoven fabrics. The model is used to demonstrate the effect of changes in heterogeneity on the form of the stress-strain curve, the existence of size effects and the possibility of localized failure. Results support the view that the model will be useful in a further investigation of behavior under more general deformation conditions.