1. Intracellular recordings were performed in neurones within the basal forebrain of guinea-pig brain slices. Following injection of biocytin (or biotinamide), a subset of recorded neurones which displayed distinct intrinsic membrane properties were confirmed as being cholinergic by immunohistochemical staining for choline acetyltransferase (ChAT). They were all located within the nucleus basalis magnocellularis. The response of the cholinergic cells to NMDA and to the agonists of the other glutamate receptors was tested by bath application of NMDA, t-ACPD, AMPA and kainate. 2. When depolarized from a hyperpolarized level, cholinergic basalis neurones display the intrinsic ability to discharge in rhythmic bursts that are generated by low-threshold Ca2+ spikes. In control solution, these rhythmic bursts were not sustained for more than 5-6 cycles. However, in the presence of NMDA when the membrane was held at a hyperpolarized level, low-threshold bursting activity was sustained for prolonged periods of time. This activity could be reversibly eliminated by D(-)-2-amino-5-phosphonopentanoic acid (D-AP5), showing that it depended upon specific activation of NMDA receptors. 3. NMDA-induced, voltage-dependent, rhythmic depolarizations persisted in the presence of tetrodotoxin (TTX), indicating that they did not depend upon a TTX-sensitive Na+ current and were generated postsynaptically. The rhythmic depolarizations were, however, eliminated by the partial replacement of Na+ with choline, demonstrating that they did depend upon Na+, the major carrier of the NMDA current. 4. In the presence of TTX, the NMDA-induced rhythmic depolarizations were also eliminated by removal of Ca2+ from or addition of Ni2+ to the bath, indicating that they also depended upon Ca2+, which is carried by both the NMDA current and the low-threshold Ca2+ current. The duration of the rhythmic depolarizations was increased in the presence of apamin, suggesting that the repolarization of the cells depended in part upon a Ca(2+)-activated K+ (SK) conductance, but that other mechanisms were additionally involved in the repolarization phase of the bursting. 5. In both the absence and presence of TTX, the NMDA-induced rhythmic activity persisted when Mg2+ was removed from the medium, indicating that the sustained rhythmic depolarizations did not hinge upon the Mg2+ block of the NMDA channels during hyperpolarization. The voltage dependence of the NMDA-induced rhythmic depolarizations in the absence of Mg2+ appeared to be determined by the properties of the low-threshold Ca2+ spike in the cholinergic basalis neurones.(ABSTRACT TRUNCATED AT 400 WORDS)