Luteinizing hormone releasing hormone (LHRH) is a physiological modulator of neuronal excitability in bullfrog sympathetic ganglia (BFSG). Actions of LHRH involve suppression of the noninactivating, voltage-dependent M-type K+ channel conductance (gM). We found, using whole-cell recordings from these neurons, that LHRH-induced suppression of gM was attenuated by the phospholipase C (PLC) inhibitor U73122 (10 µm) but not by the inactive isomer U73343 (10 µm). Buffering internal Ca2+ to 117 nm with intracellular 20 mm BAPTA + 8 mm Ca2+ or to < 10 nm with intracellular 20 mm BAPTA + 0.4 mm Ca2+ did not attenuate LHRH-induced gM suppression. Suppression of gM by LHRH was not antagonized by the inositol 1,4,5 trisphosphate (InsP3) receptor antagonist heparin (∼ 300 µm). Preventing phosphatidylinositol-4,5-bisphosphate (PIP2) synthesis by blocking phosphatidylinositol-4-kinase with wortmannin (10 µm) or with the nonhydrolysable ATP analogue AMP-PNP (3 mm) prolonged recovery of LHRH-induced gM suppression. This effect was not produced by blocking phosphatidyl inositol-3-kinase with LY294002 (10 µm). Rundown of gM was attenuated when cells were dialysed with 240 µm di-octanoyl PIP2 or 240 µm di-octanoyl phosphatidylinositol-3,4,5-trisphosphate (PIP3) but not with 240 µm di-octanoyl phosphatidylcholine. LHRH-induced gM suppression was competitively antagonized by dialysis with 240 µm di-octanoyl PIP2, but not with di-octanoyl phosphatidylcholine. These results would be expected if LHRH-induced gM suppression reflects a PLC-mediated decrease in plasma membrane PIP2 levels.