Voltage-dependent K+ currents, IA and ICa2+-K+, across the soma membrane of the Hermissenda Type B photoreceptor, have been shown to remain reduced during retention of classically conditioned behavior. IA and ICa2+-K+ undergo prolonged reduction due to [Ca2+]i elevation produced by a single pairing of a light step with a command depolarization or by iontophoretic injection of Ca2+. One pathway which could contribute to the conversion of transient Ca2+-mediated reduction of K+ currents to the persistent reduction observed with conditioning is that involving C-kinase. To examine the role of C-kinase in the long-term regulation of K+ currents, isolated Type B somata were exposed to at least 25-30 minutes' incubation in artificial sea water (ASW) containing the C-kinase activators 1-oleoyl-2-acetyl-glycerol (OAG) or 12-deoxyphorbol 13-isobutyrate 20-acetate (DPBA) or control substances [e.g., distearyolglycerol (DiSG)]. After exposure to activator (but not to control solutions) and voltage-clamp conditions which caused elevation of cytosolic Ca2+, reductions of IA and ICa2+-K+ were observed which did not reverse (up to 3 hr), even after the activator was removed. Without conditions which induced elevation of cytosolic calcium prolonged incubation with the C-kinase activators had no effect on the membrane currents. Similar exposure of homogenates of the Hermissenda nervous system to OAG and Ca2+ caused enhanced phosphorylation of specific proteins, indicating the presence of C-kinase in the Hermissenda nervous system.