  We have revealed new features in the out-of-plane resistivity rho_c of
heavily underdoped RBa_2Cu_3O_{6+x} (R=Tm,Lu) single crystals, which give
evidence for two distinct mechanisms contributing the c-axis transport. We have
observed a crossover towards "metal-like" (d rho_c/d T > 0) behavior at the
temperature T_m which quickly increases with decreasing doping. The
"metal-like" conductivity contribution dominates at T < T_m and provides a
saturation of the resistivity anisotropy, rho_c / rho_{ab}. The
antiferromagnetic ordering is found to block this "metal-like" part of the
c-axis conductivity and complete decoupling of CuO_2 planes, which may be the
reason of superconductivity disappearance.
