Most work, to date, on analysis of effects of earthquakes on dams treats dam and reservoir as two uncoupled systems. The importance of interaction between the two systems in determining the water pressure on dams during earthquakes is investigated herein. The analysis presented is approximate in nature and is aimed at exploring the problem. The earthquake acceleration is first modeled as a stationary gaussian white noise process. For this excitation, certain qualitative conclusions concerning importance of interaction effects are derived. Hydrodynamic responses of typical systems subjected to El Centro (1940) earthquake are then determined to confirm these conclusions. On basis of these results, it appears that the interaction between reservoir and dam has significant influence on the hydrodynamic responses during earthquake excitations.