We propose possible mechanisms for the highly enhanced electrooptic response by formation of J-aggregates observed for porphyrin molecules. For J-aggregates, the signal intensity deviates from the quadratic dependence on the electric field to show saturation behavior for a high field, while that for monomers obeys the quadratic dependence, as expected for the Kerr response. This phenomenon can be explained by the molecular rearrangement model, where the dipole-dipole interaction energy is modified by field-induced change in the angle between the molecular transition dipole moment and the aggregation axis. A possible contribution of charge-transfer states to the electrooptic response is also evaluated. (C) 2009 Elsevier B.V. All rights reserved.