  Shubnikov - de Haas oscillations were measured in single crystals of highly
metallic antiferromagnetic SmAgSb$_{2}$ and ferromagnetic CeAgSb$_{2}$ using a
tunnel diode resonator. Resistivity oscillations as a function of applied
magnetic field were observed via measurements of skin depth variation. The
effective resolution of $\Delta\rho\simeq20$ p$\Omega$ allows a detailed study
of the SdH spectra as a function of temperature. The effects of the Sm long -
range magnetic ordering as well as its electronic structure ($4f$-electrons) on
the Fermi surface topology is discussed.
