Cold rolled thin sheet metals are typical anisotropic materials as a consequence of the production process of rolling. Anisotropy is usually described from two aspects: as plane anisotropy and as a normal one. Most frequently used anisotropy parameter is the normal anisotropy ratio or the r value, which represents strains ratio in sheet metal plane and along its thickness. It is determined experimentally, usually as mean constant numeric valu, without testing the possible variation in dependence on realized plastic strain. Accurate defining of the r value has great significanc, since it is not only the formability parameter, but also the crucial anisotropy parameter in software for simulation of the plastic forming process. The paper presents the results of experimental investigation of three typical materials: low carbon Al killed steel DC04 sheet metal, alloy AlMg4.5Mn0.7 sheet metal and stainless steel X5CrNi18-10 sheet metal. Particular attention is given to defining the dependence of the r value on plastic strain during the forming process. The experiment requires sophisticated devices and accurate calculations. In the actual case, the computerized measuring system Zwick/Roell Z 100 was used. In addition, the paper contains the proposal on how to take into account variations of the r values during the plastic forming process.