  We present sensitive, high dynamic range, Very Long Baseline Array (VLBA)
polarimetric observations of the cores of three powerful radio galaxies: 3C166,
3C236 and 3C390.3. Significant polarization is detected in one source (3C166)
allowing us to map out the Faraday Rotation Measure (RM) distribution and
projected magnetic field direction. The inner jet of 3C166 is found to have a
rest frame RM of -2300 rad/m**2, similar to those found in quasars cores. No
polarized flux was detected from the other two sources, but in both counterjets
are seen. The counterjet in 3C236 was previously known, but the detection in
3C390.3 is a new discovery. We suggest that the low fractional polarization in
radio galaxy cores is the result of Faraday depolarization by ionized gas
associated with the accretion disk. The lower polarization of radio galaxy
cores compared to quasars is then naturally explained by unified models as a
result of the viewing angle.
