Activations of the N-methyl-D-aspartate (NMDA) receptor by glutamate were studied in outside-out patches from CA1 cells in rat hippocampal slices. Very low glutamate concentrations (20-100 nM) were used so that individual receptor activations would be well separated. The shut-time distribution contained at least five components, only the longest component being obviously concentration dependent. The three briefest shut-time components had time constants of 56 microseconds, 0.68 ms and 10.1 ms; all of these were independent of glutamate concentration. An individual activation of the receptor therefore produces a long cluster of channel openings that contains longer gaps than have been reported for receptor activations by other fast neurotransmitters. In addition, (i) some activations may contain still longer (mean 78 ms) shut periods generating 'super clusters', and (ii) a significant amount of NMDA current may be carried by prolonged ('high P(open)') periods during which the channel is open for most of the time. Such periods occur intermittently even at these very low glutamate concentrations. It is suggested that the slow time course of the NMDA receptor-mediated synaptic currents may be determined mainly by the channel activation kinetics.