The N-methyl- scD-aspartate (NMDA)-subtype of glutamate receptor is thought to play a key role in complex physiological processes such as learning, memory, and synaptic plasticity and has been implicated in pathophysiological conditions such as stroke, Alzheimer's disease, and epilepsy. Therefore, the development of selective NMDA receptor antagonists that can be used for treatment of diseases of the central nervous system involving this receptor has important clinical implications. The purpose of this study was to characterize the interactions of a novel series of bis-9-aminoacridines (1,2-propane-bis-9,9{dollar}\sp\prime {dollar}-aminoacridine, 1,2-PAA; 1,3-propane-bis-9,9{dollar}\sp\prime{dollar}-aminoacridine, 1,3-PAA; and 1,4-butane-9,9{dollar}\sp\prime{dollar}-aminoacridine, 1,4-BAA) and tetrahydro-9-aminoacridine (THA) with the NMDA receptor. Using single-channel and whole-cell patch-clamp electrophysiological techniques, the effects of these 9-aminoacridines were studied on NMDA-activated currents recorded from cultured fetal rat hippocampal neurons. The results indicated that the novel bis-9-aminoacridines and THA are open-channel blockers of this receptor. At {dollar}-{dollar}80 mV, the blocking rate constants were found to be {dollar}\rm1.1\times10\sp8\ M\sp{lcub}-1{rcub}s\sp{lcub}-1{rcub},\ 1.8\times10\sp8\ M\sp{lcub}-1{rcub}s\sp{lcub}-1{rcub},{dollar} and {dollar}\rm1.4\times10\sp8\ M\sp{lcub}-1{rcub}s\sp{lcub}-1{rcub}{dollar} for 1,2-PAA, 1,3-PAA, and l,4-BAA, respectively, compared to {dollar}\rm3.5\times10\sp7\ M\sp{lcub}-1{rcub}s\sp{lcub}-1{rcub}{dollar} for THA. The unblocking rates of the bis-9-aminoacridines were approximately 7 s{dollar}\sp{lcub}-1{rcub},{dollar} while the unblocking rate of THA was approximately 6200 s{dollar}\sp{lcub}-1{rcub}.{dollar} Consistent with the literature, Mg{dollar}\sp{lcub}2+{rcub}{dollar} was also found to act as an open-channel blocker of the NMDA receptor with blocking and unblocking rate constants of {dollar}\rm6.3\times10\sp7\ M\sp{lcub}-1{rcub}s\sp{lcub}-1{rcub}{dollar} and 970 s{dollar}\sp{lcub}-1{rcub},{dollar} respectively at {dollar}-{dollar}80 mV. In the presence of Mg{dollar}\sp{lcub}2+{rcub}{dollar} (5.4 {dollar}\mu{dollar}M), the interaction of the 9-aminoacridines with the NMDA receptor could not be described by a mechanism of open-channel blockade, because the blockade did not follow the predictions of a simple bimolecular process. Testing of various models led to the suggestion that Mg{dollar}\sp{lcub}2+{rcub}{dollar} altered the interaction by occupying the receptor's ion channel during the blockade by the 9-aminoacridine (i.e., simultaneous occupation). Using the whole-cell, patch-clamp technique and a multi-barrel, fast perfusion system, the kinetic constants of the blockade by the 9-aminoacridines were determined by a perturbation approach. From these kinetic constants, the K{dollar}\sb{lcub}\rm d{rcub}{dollar}s were estimated to be 2.3, 2.2, 2.6, and 22.3 {dollar}\mu{dollar}M for 1,2-PAA, 1,3-PAA, 1,4-BAA, and THA, respectively. In the presence of Mg{dollar}\sp{lcub}2+{rcub}{dollar} (100 {dollar}\mu{dollar}M), the unblocking rate of 1,2-PAA was faster and increased the K{dollar}\sb{lcub}\rm d{rcub}{dollar} to 3.8 {dollar}\mu{dollar}M, which was consistent with the simultaneous occupation model. All of the 9-aminoacridines were less effective in blockade of kainate- and quisqualate-activated currents. In summary, this study demonstrated that: (1) the bis-9-aminoacridines are potent open-channel blockers of the NMDA receptor having affinities an order of magnitude higher than THA; (2) Mg{dollar}\sp{lcub}2+{rcub}{dollar} also blocks NMDA-activated currents by a mechanism that can be best described as open-channel blockade that is not strictly sequential; and (3) the interactions of the 9-aminoacridines with the channel of the NMDA receptor are not prevented by the presence of Mg{dollar}\sp{lcub}2+{rcub}{dollar} within the channel, but Mg{dollar}\sp{lcub}2+{rcub}{dollar} does reduce the affinity of this interaction.