  We have studied the thermopower of La_2CuO_{4+z} and Nd_2CuO_{4-y} which
undergo an antiferromagnetic transition near room temperature and
Bi_2Sr_{2-x}La_xCuO_{6+z} for which a broad spectrum of doping is possible. The
thermopower of La_2CuO_{4+z} and Nd_2CuO_{4-y} is seen to exhibit an anomaly at
the Neel temperature, whereas the resistivity is not. The extrapolated
zero-temperature intercept of the thermopower, which is known to be positive
for hole-doped cuprate superconductors, is found to become negative for
Bi_2Sr_{2-x}La_xCuO_{6+z} above some critical doping-level. The data strongly
suggest that the thermopower of high-T_c cuprates contains a large amount of
extra contribution in addition to the usual diffusion thermopower. We discuss
origins of the extra contribution in the thermopower.
