Regulation of the activity of a cloned component of a voltage-activated K+ channel, Kv1.5, was studied by expressing the K+ channel and receptors for platelet-derived growth factor (PDGF) or fibroblast growth factor (FGF) simultaneously in Xenopus oocytes. Receptor activation mediated a decline in the Kv1.5 current amplitude, with a half-time of about 20 min. The reduction in K+ current amplitude occurred with little change in the kinetics or voltage sensitivity of activation. A similar phenomenon was found when the human thrombin or rat 5-HT1c receptors, two receptors that increase phospholipase C activity, were tested in coexpression experiments. A mutant FGF receptor, which does not activate phospholipase C-gamma 1 but retains several of its other functions, did not modulate the Kv1.5 current. Simultaneous injection of inositol trisphosphate and superfusion of phorbol 12-myristate 13-acetate reproduced the modulation of the Kv1.5 current. These results demonstrate that the PDGF and FGF receptors can modulate a voltage-activated K+ channel by increasing phospholipase C activity, and suggest that PDGF or FGF may be able to alter rapidly the electrical excitability of neurons.