  Below the Kondo temperature the heavy Fermion compound YbRh$_{2}$Si$_{2}$
shows a well defined Electron Spin Resonance (ESR) with local Yb$^{3+}$
properties. We report a detailed analysis of the ESR intensity which gives
information on the number of ESR active centers relative to the ESR of well
localized Yb$^{3+}$ in YPd$_3$:Yb. The ESR lineshape is investigated regarding
contributions from itinerant centers. From the ESR of monoisotopic
$^{174}$YbRh$_{2}$Si$_{2}$ we could exclude unresolved hyperfine contributions
to the lineshape.
