While Ca++ influx is essential for the activation of the cell cycle machinery, the processes that allow Ca++ to enter the cell have not been clearly elucidated. Electrophysiologic and molecular studies have identified multiple Ca++ channel genes expressed in mammalian cells. CaV3.x gene family members, encoding low voltage-activated or T-type Ca++ channels, were first identified in the CNS and subsequently in nonneuronal tissue. There are conflicting reports describing a potential role for T-type Ca++ channels in controlling the cell cycle and proliferation. In the present study, using a whole-cell patch clamp, quantitative reverse transcriptase polymerase chain reaction, and immunocytochemistry we found that CaV3.1 was the predominant CaV3.x channel expressed in early passage human pulmonary artery cells in vitro and in the media of human pulmonary arteries in vivo. CaV3.x messenger RNA expression was highest on reentry to the cell cycle. Selective blockade of CaV3.1 expression with small-interfering RNA and pharmacologic blockade of T-type currents completely inhibited proliferation in response to a 5% serum and prevented cell-cycle entry.