Investigations of recent near source earthquake damages indicated that the vertical excitation played a significant role in the structural behavior and in some cases was the main reason of the damage or failure. However, most of the building codes put more emphasis on the horizontal excitation and do not properly consider the vertical excitation. In this paper, the nonlinear response of a ten-storey R.C. building frame subjected to combined horizontal and vertical excitations was investigated. The nonlinear dynamic analysis computer program IDARC4 was utilized for the analysis. Two different levels of horizontal peak ground acceleration and three different levels of vertical peak ground acceleration were considered for the combination. The results indicated that the vertical excitation strongly increased the columns compression forces, especially for the interior and the upper storey columns. These forces were increased to be three or four times as much as the original static forces. The strong vertical excitation caused about 70% of the total compression force. Unlike the horizontal excitation, the strong vertical excitation resulted in high axial tension forces in columns. The bending moments and the shearing forces in the upper storey columns increased with the increase of the vertical excitation. The level of vertical excitation itself had stronger effect than the ratio of the vertical peak to the horizontal peak. It was also concluded that the internal forces were greatly underestimated if the same spectral shape was used for both horizontal and vertical excitations.