The single channel properties of neuronal nicotinic ACh receptors (nAChRs) were investigated in outside-out patches from CA1 stratum radiatum interneurons from thin slices of rat hippocampus. The application of ACh (10 microM to 1 mM) induced the opening of observable single channel currents with two distinct current levels, which corresponded to conductance levels of 38 +/- 3 and 62 +/- 2 pS. The 38 pS channel was observed in 10 % (n = 260) of patches, whereas the 62 pS channel was observed in 4 % of patches; these two channel types were most often observed independently. The alpha7-selective nAChR antagonist methyllycaconitine (MLA; 50 nM) reduced the open probability of the 38 pS channel by 73 %. In contrast, the 62 pS channel was unaffected by MLA, but instead was blocked by dihydro-beta-erythroidine (DHbetaE; 10 microM), a broad spectrum nAChR antagonist. These data suggest that rat hippocampal CA1 stratum radiatum interneurons in the slice possess at least two different types of functional nAChRs, an alpha7-containing subtype with a single channel conductance of 38 pS, and a non-alpha7 subtype with a single channel conductance of 62 pS.