  We present an experimental study of Z-scan measurements of the nonlinear
response of cold Cs atoms in a magneto-optical trap. Numerical simulations of
the Z-scan signal accounting for the nonuniform atomic density agree with
experimental results at large probe beam detunings. At small probe detunings a
spatially varying radiation force modifies the Z-scan signal. We also show that
the measured scan is sensitive to the atomic polarization distribution.
