Polyamines produce on the NMDA-receptor channel activity a regulatory effect subsequent to their binding to specific sites on the receptor-complex. The photoactivatable polyamine derivative L-azidophenylspermine shows properties which suggest its potential as a photoaffinity probe to investigate the nature and topology of these sites. In the dark, its effect on the binding of tritiated N-(1-[thienyl]cyclohexyl)piperidine ([3H]TCP) to synaptosomal plasma membranes is similar to that of diaminodecane. Arcaine antagonizes both the effects of L-azidophenylspermine and diaminodecane on [3H]TCP binding. L-Azidophenylspermine competes in post-synaptic densities with tritiated spermidine for a unique binding site with an EC50 similar to that of spermine. Upon irradiation, L-azidophenyl-spermine incorporates into this material with a high efficiency to a level consistent with both the Bmax for tritiated spermidine and the estimated density of NMDA receptors in this fraction.