Effects of intracellularly injected activators of protein kinase C on the InsP3-induced K+ current and the Ca2+-activated K+ current recorded from identified neurons (R9-R12) of Aplysia kurodai were investigated with conventional voltage-clamp and pressure-injection techniques. Intracellular injection of InsP3 into identified neurons produced a 4-aminopiridine (4-AP)-resistant, tetraethylammonium (TEA)-sensitive, and quinidine-sensitive K+ current similar to the Ca2+ activated K+ current elicited by direct injection of Ca2+ ions into the same neurons. The diacylglycerol analogue 1,2-oleoylacetylglycerol (OAG) at an intracellular concentration of 65 nM produced irreversible decreases in both the InsP3-induced K+ current and the Ca2+-activated K+ current. The phorbol 12,13-dibutyrate (PDBu) at an intracellular concentration of 150 nM also decreased irreversibly both the InsP3-induced K+ current and the Ca2+-activated K+ current. These results suggest that protein kinase C activators reduce both the InsP3-induced K+ current and the Ca2+-activated K+ current recorded from certain identified neurons of Aplysia and that protein kinase C reduces the ability of Ca2+ to open K+ channels rather than affecting the ability of InsP3 to release Ca2+ from intracellular stores.