The present study attempts to model the load-deflection behaviour of slender rectangular reinforced concrete beams, accounting for deflections in both the vertical and horizontal planes under gravity loading. The lateral deflection depends on imperfections and proximity of the applied moment to the critical buckling moment of the beam. A simplified formulation is proposed here, based on an extension of existing theories that model the reduction in flexural stiffness and torsional stiffness with increasing load, for the purpose of predicting the complete load-deflection response of slender beams. This formulation is found to generate load-deflection plots that match reasonably with the experimental results.