We study, theoretically, the transient electric birefringence of semidilute and concentrated solutions of self-assembling rodlike aggregates in the fast reaction regime. The aggregates interact via a hard core excluded volume potential. We find that although the rods are very polydisperse, the zero-field relaxation is characterized by at most three exponentials depending on whether or not the assemblies have a permanent dipole moment. The static response, and one of the relaxation times, diverges at the nematic spinodal.