The development of a conceptual framework to test different viscoelastic constitutive laws is presented. This framework has the advantage of satisfying a priori the thermodynamic restrictions and is valid for large deformations. In addition, the different mechanical contributions are separated according to the time scale of their effects. As an illustration of its ability to model the immediate, short time memory and long time memory contributions, the framework is used to identify mechanical tests performed on human patellar tendons. The resulting viscoelastic law is then proposed to model the soft biological tissues as these tissues present an important viscoelastic behaviour. (C) 2000 Editions scientifiques et medicales Elsevier SAS.