Contraction stresses generated in restoratives during setting are among the major problems in adhesive dentistry, since they often result in loss of adhesion from the cavity walls or in post-operative pain. The rate of stress development and the ultimate magnitude of the stress, which determine the seriousness of these problems, depend on the relatively unknown visco-elastic behavior of the restoratives during setting. The aim of this study was to determine the visco-elastic parameters during setting, to aid our understanding of the process of contraction stress development. A dynamic mechanical method was used in which the materials were subjected to periodic strain cycles in a universal testing machine during the first 60 min of setting. The visco-elastic parameters (viscosity eta and Young's modulus E) were calculated by analysis of the experimental stress-strain data with a simple mechanical model according to Maxwell. Two restorative materials from different classes were investigated: a two-paste resin composite and a conventional glass-ionomer cement. A comparison of the results showed significant differences in the development of viscosity and stiffness in the early stage of setting. The resultant relaxation time (eta/E) of the glass ionomer remained at a low level during the first 15 min, whereas that of the resin composite increased markedly. This is of clinical importance, since it implies that, during the early setting stage, glass ionomers are better capable of reducing the contraction stresses than resin composites, thus increasing the likelihood that the bond with the cavity walls will form and survive during setting.