GH 3 pituitary cells have high tendency to exhibit spontaneous Ca 2 + action potential and its frequency is increased by treatment of thyrotropin-releasing hormone (TRH). Although spontaneous Ca 2 + firing was thought to be a significant for the induction of oscillations in cytosolic Ca 2 + concentrations ([Ca 2 + ]i), no attempt to elucidate the mechanism was done so far. We now demonstrate that in GH 3 cells spontaneous Ca 2 + firing frequency was increased 2 or 3 times higher, comparable to that for TRH, by the injection of guanosine 5 -0-3-thiotriphosphate (GTP γ S), rab3A effector domain peptide and phorbol-dibutyrate (PDBu), whereas guanosine 5 -O-(2-thiodiphosphate) (GDP β S), H-rab5 peptide and 4α phorbol injection did not. The present studies suggest that the spontaneous Ca 2 + firing rate will be regulated by changes in K + channel conductance, which could be controlled by phosphorylated small molecular weight G protein by protein kinase C.