Modulation of a cloned Aplysia K channel, AKv1.1a, by protein kinase C (PKC) activators was examined in Xenopus oocytes expression system. Following the application of phorbol esters (phorbol 12-myristate 13-acetate, PMA; phorbol 12,13-dibutyrate, PDBu), or a diacylgrycerol analogue (1-oleoyl-2-acetyl-sn-glycerol, OAG), the fast inactivation of the AKv1.1a became slower and the peak current increased (up-modulation). However, the effect was transient. The expressed current was decreased even below control level about 15 to 20 min after the treatment (down-modulation). Both effects by PMA was blocked by the kinase inhibitor, H7, suggesting that phosphorylation by PKC is involved. The amino acid sequence of AKv1.1a contains three putative phosphorylation sites by PKC (Ser²⁴, Thr³⁴⁵, Ser³⁴⁹). We tested their contributions to the PMA-induced modulation by site-directed mutagenesis. The results suggest that the up-modulation by PKC activators is due to the inhibition of the fast inactivation by the amino-terminal domain (N-type inactivation), thereby increase the time the channels are conductive. Phosphorylation of Ser²⁴ may enhance the PKC-induced down-modulation, while phosphorylation of Thr³⁴⁵ may inhibit the down-modulation. By contrast, mutation of Ser³⁴⁹ did not affect the modulation. The N-type inactivation were not indispensable for the down-modulation because the amino-terminal deletion mutant also showed some down-modulation although its onset was quite slow. Thus, the down-modulation of AKv1.1a may be heterogeneous. Because some modulation was still observed even in a mutant which lacks all putative phosphorylation sites mentioned above, additional mechanisms such as the regulation by other phosphorylated protein(s) exist endogenously in oocytes and/or recruitment of other kinases by PKC-activation may also be involved in the observed modulation of the AKv1.1a.