The current-voltage relationshipI(V) for several different ferroelectric thin films often exhibits an apparent negative differential resistivity. In the present paper we show that in lead zirconate-titanate (PZT) this is a spurious artifact arising from rapid (parametric) measurement techniques and that it disappears for d.c. measurements made with measuring times ≥300 s. However, in other ferroelectric materials (selected samples with unusually high trap densities) it appears to be a genuine effect characterized by anI= aV3current-voltage relationship at high current densities (≥100 nA/cm2),I(V) hysteresis above a “pseudo-breakdown voltage,” and distinctV2-to-V3cross-over threshold.