1. Substance P (SP) induces a slow neuronal excitation in cholinergic neurons from the nucleus basalis by suppressing an inwardly rectifying K+ current (Kir). We have determined which G protein alpha-subunit mediates this SP effect. 2. After intracellularly injecting antibody against each alpha-subunit of G proteins (Gq alpha/11 alpha, G12 alpha, and G13 alpha) with an Eppendorf microinjector, we examined, by using the whole cell patch-clamp and the ON-cell mode of single-channel recording, the effect of SP on Kir in cultured neurons of the nucleus basalis. The effect of SP on Kir was substantially reduced in neurons injected with antibodies to Gq alpha/11 alpha but not with antibodies to G12 alpha or G13 alpha. 3. The effects of antibodies against three isozymes of phospholipase C (PLC-beta 1, PLC-beta 2, and PLC-beta 3) were tested. The SP-induced suppression of Kir was reduced by antibody against PLC-beta 1 but not by antibodies against PLC-beta 2 or PLC-beta 3. 4. We conclude that the SP-induced inhibition of Kir in nucleus basalis neurons is mediated by Gq/11 and PLC-beta 1.