Plasma membrane potassium (K+) channels are required for tumor cell proliferation and apoptosis. However, the signal transduction mechanisms underlying K+ channel-dependent tumor cell proliferation or apoptosis remains elusive. Using HeLa and A2780 cells as study models, we tested the hypothesis that apoptotic proteins are linked with K+ channel-dependent tumor cell cycle and apoptosis. The patch-clamping study using the whole-cell mode revealed two components of voltage-gated outward K+ currents: one is sensitive to either tetraethylammonium (TEA) or tetrandrine (Tet), a maxi-conductance Ca2+-activated K+ (BK) channel blocker, and the other is sensitive to 4-aminopyridine (4-AP), a delayed rectifier K+ channel blocker. MTT and flow cytometry assays showed that TEA, Tet, or iberiotoxin (Ibtx), a selective BK channel blocker, inhibited HeLa and A2780 cell proliferation in a dose-dependent manner with G1 phase arrest. Pretreatment with TEA or Tet also induced apoptosis in HeLa and A2780 cells. However, glibenclamide (Gli), an ATP-sensitive K+ channel blocker, did not influence K+ currents, proliferation or apoptosis. Western blot analyses showed that while pretreatment of TEA and Tet produced an increase in expressions of p53, p21, and Bax, pretreatment of these two agents led to a decrease in expressions of heat shock protein (hsp)90alpha, hsp90beta, and hsp70. Our results indicate that the blockade of BK channels results in tumor cell apoptosis and cycle arrest at G1 phase, and the transduction pathway underlying the anti-proliferative effects is linked to the increased expression of apoptotic protein p53 and the decreased expression of its chaperone proteins hsp.