A theoretical analysis of the bending of the rotor blade in the plane of rotation is given with consideration of the elasticity of the blade. The analysis also includes the bending of a blade with variable mass distribution along the span. The results lead to matrix equations tha t are convenient for arithmetic calculation. The application of this theoretical analysis is illustrated by numerical calculations. The results found in the case of a typical rotor blade show tha t the maximum deflection in the plane of rotation is about one-tenth of that in the flapping plane and tha t the maximum bending moment in the plane of rotation is about one-fourth of that in the flapping plane. When a blade with variable section is compared with one with an equivalent constant blade section, the bending moment for the variable section is found to be appreciably greater than that for the equivalent constant section in the region of the inner half span.