A slowly activating, voltage-dependent potassium channel protein cloned from rat kidney was expressed in Xenopus oocytes. Two activators of protein kinase C, 1-oleoyl-2-acetyl-rac-glycerol and phorbol 12,13-didecanoate, inhibited the current. This inhibition was blocked by the kinase inhibitor staurosporine. Inhibition of the current was not seen in channels in which Ser103 was replaced by Ala, although other properties of the current were unchanged. These results indicate that inhibition of the potassium current results from direct phosphorylation of the channel subunit protein at Ser103.