To explore the athermal effect of an external electrostatic field on the stabilities of protein conformations and the binding affinities of protein-protein/ligand interactions, the dependences of the polar and hydrophobic interactions on the external electrostatic field, -E-ext, were studied using molecular dynamics (MD) simulations. By decomposing E-ext into, along, and perpendicular to the direction formed by the two solutes, the effect of E-ext on the interactions between these two solutes can be estimated based on the effects from these two components. E-ext was applied along the direction of the electric dipole formed by two solutes with opposite charges. The attractive interaction free energy between these two solutes decreased for solutes treated as point charges. In contrast, the attractive interaction free energy between these two solutes increased, as observed by MD simulations, for E-ext = 40 or 60 MV/cm. E-ext was applied perpendicular to the direction of the electric dipole formed by these two solutes. The attractive interaction free energy was increased for E-ext = 100 MV/cm as a result of dielectric saturation. The force on the solutes along the direction of E-ext computed from MD simulations was greater than that estimated from a continuum solvent in which the solutes were treated as point charges. To explore the hydrophobic interactions, E-ext was applied to a water cluster containing two neutral solutes. The repulsive force between these solutes was decreased/increased for E-ext along/perpendicular to the direction of the electric dipole formed by these two solutes.