Postsynaptic densities (PSDs) isolated from porcine cerebral cortices are large aggregates consisting of more than 30 different proteins. Inductively coupled plasma-mass spectrometric analyses revealed that isolated PSDs contained zinc at a concentration of 4.1 nmol per mg protein. Treatment with 8 M urea lead to dissociation of the PSDs into small components and, concomitantly, depletion of most of their bound zinc. After removal of the urea by dialysis, urea-dissociated PSD proteins did not reassemble into aggregates by themselves. Adding ZnCl 2 to urea-treated PSD samples resulted in the assembly of urea-dissociated proteins into large aggregates with morphology and protein composition closely resembling those of the original PSDs. Mg 2+ , Ca 2+ , Co 2+ , Cd 2+ , Cu 2+ , Mn 2+ , Fe 3+ , K + and Na + ions at higher concentrations also induced the aggregation of urea-dissociated PSD protein. The structures of the K + -, Na + -, Mg 2+ -and Ca 2+ -induced aggregates were distinct from that of the original PSDs. Our results indicate that the structure of the PSD could be disassembled and reassembled under in vitro conditions. They further suggest that Zn 2+ ions, by binding to certain zincbinding proteins, play an important role in the formation and maintenance of the structure of the PSD.