Lateral potential distributions in 2D electron systems are imaged unde
r magnetotransport conditions with an electrooptic method. In low mobi
lity delta-doped samples, the results agree with a classical picture i
n the ohmic regime. For high currents, a uniform breakdown is found. I
n high-mobility modulation-doped heterostructures, the change in the p
otential distribution at breakdown is strongly asymmetric upon current
reversal.