Saturation studies with [3H]ifenprodil (in the presence of 3 microM (+)-3-PPP and 10 microM GBR 12909) demonstrated the presence of a high-affinity (Kd = 0.45 microM) population of binding sites in sagittal rat brain sections. This binding was inhibited by spermine (IC50 = 69 microM) and spermidine (IC50 = 623 microM) but not by putrescine (1 mM). Ifenprodil displaced this binding in a biphasic fashion with a high affinity component (IC50 = 0.992 microM) accounting for approximately 50% of the spermine-displaceable [3H]ifenprodil binding. The polyamine-sensitive [3H]ifenprodil binding sites were heterogenously distributed in the rat brain, the highest binding densities being found in the hippocampus and in the nucleus accumbens. The anatomical distribution of [3H]ifenprodil binding sites closely matches that previously reported for the N-methyl-D-aspartate (NMDA) receptor.