Results of a parametric study on site response effects due to seismic strong ground motions are used in this paper to develop a new "dynamic foundation factor" for the National Building Code of Canada. In order to capture the effect of the site resonance, the proposed dynamic foundation factor, F*, is given as a function of the ratio between the fundamental period of the building and the site period, T/Ts (i.e., F* spectra in terms of T/Ts). The parametric study results suggested that the proposed F* spectra be dependent on four different classes of soil deposits. These classes are deep cohesive (class 1), deep cohesionless (class 2), shallow cohesive and cohesionless (class 3), and dense sand (class 4). For classes 1 and 2, the F* spectra are independent of the ratio of peak acceleration to peak velocity, av, of the seismic ground motion. For classes 3 and 4, however, the F* spectra are an increasing function of the ratio a/v. A scaling multiplier is introduced to take into account the influence of level of intensity, v. The proposed dynamic foundation factor is compared with the National Building Code of Canada 1990 foundation factor. Finally, actual sites are used to check the validity and consistency of the proposed dynamic foundation factor. Key words: seismic, foundation factor, soil, site, amplification, building, shear, force, period, spectra.