The effects of estrogen (17 beta-estradiol) on calcium current density were examined in the GH3 anterior pituitary cell line. The addition of 1 nM estrogen to the growth medium induces a 4- to 5-fold increase in low voltage-activated (LVA) Ca2+ current density without affecting high voltage-activated Ca2+ current density. The increase is significant after 24 h, reaches a maximum level in 3 days, and reverses with a similar time course when the estrogen is withdrawn. The EC50 for estrogen stimulation of LVA Ca2+ current density is about 30 pM. The LVA current induced by estrogen has similar voltage dependence and time course of activation, inactivation, and deactivation as controls. The effect of estrogen requires protein synthesis, since it is blocked by cycloheximide. It is suggested that the estrogen-induced increase in LVA Ca2+ current density may be due to an increase in the number of functional channels in the membrane. This effect could be due to insertion of new channels or synthesis of a protein that converts preexisting silent channels into functional channels.