This work experimentally confirms the pathway of activation of Ha-ras-p21, which was calculated by the method of Targeted Molecular Dynamics (TMD) (Diaz JF, Wroblowski B, Schlitter J, Engelborghs Y, 1997a, Proteins Struct Funct Cenet 28:434-451). The process can be studied experimentally by analyzing the binding of BeF3- to the GDP complex of the active fluorescent mutant Y32W (Diaz JF, Sillen A, Engelborghs Y, 1997b, J Biol Chem 227:23138-23143). Two mutants, V29G and I36G, have been constructed at both sides of the effector loop of the active fluorescent mutant. This was done to check the proposed reaction pathway and to provide further insight into the mechanism of the, activation of ms proteins. Both mutations accelerate the conformational isomerization with two orders of magnitude, demonstrating convincingly the role of these residues as hinges of the effector loop in one or more of the transitions of the conformational change. These results provide experimental support to the pathway calculated by TMD analysis.